Online Etymology Dictionary

month (n.): Old English monað, from Proto-Germanic *menoth- (cognates: Old Saxon manoth, Old Frisian monath, Middle Dutch manet, Dutch maand, Old High German manod, German Monat, Old Norse manaðr, Gothic menoþs "month"), related to *menon- "moon" (see moon (n.); the month was calculated from lunar phases). Its cognates mean only "month" in the Romance languages, but in Germanic generally continue to do double duty. Phrase a month of Sundays "a very long time" is from 1832 (roughly 7 and a half months, but never used literally).
twelve-month (n.): "a year," Old English twelf-monð; see twelve + month.
monthly (adv.): 1530s, from month + -ly (2). As an adjective from 1570s. Old English had monaþlic, but the modern words seem to be separate formations.
semi-monthly (adj.): also semimonthly, 1809, from semi- + monthly.
bimonthly (adj.): also bi-monthly, 1846, "happening once in two months, every two months," also "occurring twice a month," a hybrid from bi- + monthly.
flavor (n.): c. 1300, "a smell, odor" (usually a pleasing one), from Old French flaor "smell, odor; action of smelling, sense of smell," probably from Vulgar Latin flator "odor," literally "that which blows," in classical Latin "blower," from flare "to blow, puff," which is cognate with Old English blawan (see blow (v.1)).

"Not common before Milton's time" [Century Dictionary], and it is not clear what exactly Milton meant when he used it. The same Vulgar Latin source produced Old Italian fiatore "a bad odor." Sense of "taste, savor" is 1690s, perhaps 1670s; originally "the element in taste which depends on the sense of smell." The -v- in the English word is euphonic or perhaps from influence of savor. Flavor-of-the-month is from 1946 (originally of ice cream).
sere (adj.): Old English sear "dried up, withered, barren," from Proto-Germanic *sauzas (cognates: Middle Low German sor, Dutch zoor "dry"), from PIE root *saus- "dry" (cognates: Sanskrit susyati "dries, withers;" Old Persian uška- "dry" (adj.), "land" (n.); Avestan huška- "dry;" Greek auos "dry," auein "to dry;" Latin sudus "dry"). A good word now relegated to bad poetry. Related to sear. Sere month was an old name for "August."
autumn (n.): late 14c., autumpne (modern form from 16c.), from Old French autumpne, automne (13c.), from Latin autumnus (also auctumnus, perhaps influenced by auctus "increase"), which is of unknown origin. Perhaps from Etruscan, but Tucker suggests a meaning "drying-up season" and a root in *auq- (which would suggest the form in -c- was the original) and compares archaic English sere-month "August."

Harvest was the English name for the season until autumn began to displace it 16c. In Britain, the season is popularly August through October; in U.S., September through November. Compare Italian autunno, Spanish otoño, Portuguese outono, all from the Latin word. Unlike the other three seasons, its names across the Indo-European languages leave no evidence that there ever was a common word for it.

Many "autumn" words mean "end, end of summer," or "harvest." Compare also Lithuanian ruduo "autumn," from rudas "reddish," in reference to leaves; Old Irish fogamar, literally "under-winter."
menstrual (adj.): late 14c., "pertaining to menses," also (in astronomy) "monthly," from Old French menstruel, from Latin menstrualis "monthly," especially "of or having monthly courses," from menstruus "of a month, every month, monthly, pertaining to a month," from mensis "month" (see moon (n.)).
mensal (adj.1): "monthly," 1860, from Latin mensis "month" (see moon (n.)) + -al (1).
ultimo (adv.): "in the month preceding the present," 1610s, common in abbreviated form ult. in 18c.-19c. correspondence and newspapers, from Latin ultimo (mense) "of last (month)," ablative singular masc. of ultimus "last" (see ultimate). Earlier it was used in the sense of "on the last day of the month specified" (1580s). Contrasted with proximo "in the next (month)," from Latin proximo (mense).
menarche (n.): 1896, from German menarche (1895), from Greek men (genitive menos) "month" (see moon (n.)) + arkhe "beginning" (see archon).
menses (n.): "monthly discharge of blood from the uterus," 1590s, from Latin menses, plural of mensis "month" (see moon (n.)).
menstruous (adj.): 1530s, from French menstrueus, from Latin *menstruosus, from menstruum, from menstruus (adj.) "monthly," from mensis "month" (see moon (n.)).
Thammuz (n.): 1530s, from Hebrew tammuz, tenth month of the Jewish civil year, fourth of the sacred, covering parts of June and July; also the name of a Syrian deity equivalent to Phoenician Adon, whose festival began with the new moon of this month (compare Tammuz).
brumal (adj.): "belonging to winter," 1510s, from Latin brumalis, from bruma "winter" (see brume). The Latin word also is the ultimate source of Brumaire, second month (Oct. 22-Nov. 20) in calendar of the French Republic, literally "the foggy month," coined 1793 by Fabre d'Eglantine from French brume "fog" (see brume).
amenorrhea (n.): 1804, Modern Latin, from Greek privative prefix a- "not" (see a- (3)) + men "month" (see moon (n.)) + rhein "to flow" (see rheum). Related: amenorrheal.
intercalary (adj.): "inserted into the calendar," 1610s, from Latin intercalarius "intercalary, of an intercalary month," from intercalare "proclaim an intercalary day" (see intercalate). General sense of "interpolated" is attested from 1798.
Ramadan (n.): ninth month of the Muslim year, 1590s, from Arabic Ramadan (Turkish and Persian ramazan), originally "the hot month," from ramida "be burnt, scorched" (compare Mishnaic Hebrew remetz "hot ashes, embers"). In the Islamic lunar calendar, it passes through all seasons in a cycle of about 33 years, but evidently originally it was a summer month.
November: c. 1200, from Old French novembre and directly from Latin November (also Novembris (mensis)), from novem "nine" (see nine). The ninth month of the Roman calendar, which began in March. For -ber see December. In Old English, it was Blotmonað "month of sacrifice," literally "blood-month," the time when the early Saxons prepared for winter by sacrificing animals, which they then butchered and stored for food.
August: eighth month, 1097, from Latin Augustus (mensis), sixth month of the later Roman calendar, renamed from Sextilis (literally "sixth") in 8 B.C.E. to honor emperor Augustus Caesar, literally "Venerable Caesar" (see august (adj.)). In England, the name replaced native Weodmonað "weed month." One of two months given new names to honor Roman emperors (July being the other), the Romans also gave new imperial names to September (Germanicus) and October (Domitian) but luckily these did not stick.
May: fifth month, early 12c., from Old French mai and directly from Latin Majus, Maius mensis "month of May," possibly from Maja, Maia, a Roman earth goddess (wife of Vulcan) whose name is of unknown origin; possibly from PIE *mag-ya "she who is great," fem. suffixed form of root *meg- "great" (cognate with Latin magnus). Replaced Old English þrimilce, month in which cows can be milked three times a day. May marriages have been considered unlucky at least since Ovid's day. May-apple attested from 1733, American English.
July: seventh month, c. 1050, Iulius, from Anglo-French julie, Old French Juil, Jule (Modern French uses a diminutive, Juillet), from Latin Iulius "fifth month of the Roman calendar" (which began its year in March), renamed after his death and deification in honor of Gaius Julius Caesar, who was born in this month. In republican Rome it had been Quintilis, literally "fifth." Accented on the first syllable in English until 18c.; "the modern Eng. pronunciation is abnormal and unexplained" [OED]. Replaced Old English liða se æfterra "later mildness," from liðe "mild."
Jericho: Biblical city (Num. xxii:1, etc.), perhaps ultimately from Hebrew yareakh "moon, month," and thus a reference to an ancient moon cult. As a figurative place of retirement (17c.), the reference is to II Sam. x:5.
menstruation (n.): 1680s, from Late Latin menstruare, from menstruus "monthly" (see menstrual) + -ation. Old English equivalent was monaðblot "month-blood." Middle English had menstrue (n.), late 14c., from Old French menstrue, from Latin menstruum.
trimester (n.): 1821, "period of three months," from French trimestre (early 17c.), from Latin trimestris "of three months," from tri- "three" (see tri-) + mensis "month" (see moon (n.)). Specific obstetrics sense is attested from 1916. Related: Trimestrial.
April: fourth month, c. 1300, aueril, from Old French avril (11c.), from Latin (mensis) Aprilis, second month of the ancient Roman calendar, of uncertain origin, perhaps based on Apru, an Etruscan borrowing of Greek Aphrodite. Or perhaps *ap(e)rilis "the following, the next," from its place as the second month of the old Roman calendar, from Proto-Italic *ap(e)ro-, from PIE *apo- "away, off" (see apo-; compare Sanskrit aparah "second," Gothic afar "after"). With month-name suffix -ilis as in Quintilis, Sextilis (the old names of July and August). In English in Latin form from mid-12c. Replaced Old English Eastermonað, which was named for a fertility goddess (see Easter). Re-spelled in Middle English on Latin model (apprile first attested late 14c.).
February (n.): late 14c., ultimately from Latin februarius mensis "month of purification," from februare "to purify," from februa "purifications, expiatory rites" (plural of februum "means of purification, expiatory offerings"), which is of uncertain origin, said to be a Sabine word. De Vaan says from Proto-Italic *f(w)esro-, from a PIE word meaning "the smoking" or "the burning" (thus possibly connected with fume (n.)). The sense then could be either purification by smoke or a burnt offering.

The last month of the ancient (pre-450 B.C.E.) Roman calendar, so named in reference to the Roman feast of purification, held on the ides of the month. The Old English name for it was solmonað, which is said to mean "mud month." English first borrowed the Roman name from Old French Feverier, which yielded Middle English Feverer, Feoverel, etc. (c. 1200) before the 14c. respelling to conform to Latin.
moon (n.): Old English mona, from Proto-Germanic *menon- (cognates: Old Saxon and Old High German mano, Old Frisian mona, Old Norse mani, Danish maane, Dutch maan, German Mond, Gothic mena "moon"), from PIE *me(n)ses- "moon, month" (cognates: Sanskrit masah "moon, month;" Avestan ma, Persian mah, Armenian mis "month;" Greek mene "moon," men "month;" Latin mensis "month;" Old Church Slavonic meseci, Lithuanian menesis "moon, month;" Old Irish mi, Welsh mis, Breton miz "month"), probably from root *me- "to measure," in reference to the moon's phases as the measure of time.

A masculine noun in Old English. In Greek, Italic, Celtic, Armenian the cognate words now mean only "month." Greek selene (Lesbian selanna) is from selas "light, brightness (of heavenly bodies)." Old Norse also had tungl "moon," ("replacing mani in prose" - Buck), evidently an older Germanic word for "heavenly body," cognate with Gothic tuggl, Old English tungol "heavenly body, constellation," of unknown origin or connection. Hence Old Norse tunglfylling "lunation," tunglœrr "lunatic" (adj.).

Extended 1665 to satellites of other planets. To shoot the moon "leave without paying rent" is British slang from c. 1823; card-playing sense perhaps influenced by gambler's shoot the works (1922) "go for broke" in shooting dice. The moon race and the U.S. space program of the 1960s inspired a number of coinages, including, from those skeptical of the benefits to be gained, moondoggle (based on boondoggle). The man in the moon is mentioned since early 14c.; he carries a bundle of thorn-twigs and is accompanied by a dog. Some Japanese, however, see a rice-cake-making rabbit in the moon.
ditto: 1620s, Tuscan dialectal ditto "(in) the said (month or year)," literary Italian detto, past participle of dire "to say," from Latin dicere (see diction).

Originally used in Italian to avoid repetition of month names in a series of dates; generalized meaning of "same as above" first recorded in English 1670s. Dittohead, self-description of followers of U.S. radio personality Rush Limbaugh, attested by 1995. dittoship is from 1869.
eland (n.): Cape elk, large South African antelope, 1786, from Dutch eland "elk," probably from a Baltic source akin to Lithuanian elnias "deer," from PIE *el- (2) "red, brown" (see elk), cognate with first element in Greek Elaphebolion, name of the ninth month of the Attic year (corresponding to late March-early April), literally "deer-hunting (month)." Borrowed earlier in English as ellan (1610s, via French), ellend (from the German form of the word).
March: third month, c. 1200, from Anglo-French marche, Old French marz, from Latin Martius (mensis) "(month) of Mars," from Mars (genitive Martis). Replaced Old English hreðmonaþ, the first part of which is of uncertain meaning, perhaps from hræd "quick, nimble, ready, active, alert, prompt." For March hare, proverbial type of madness, see mad.
Pyanepsia (n.): festival in honor of Apollo on the 7th of Pyanepsion (fourth month of the Attic calendar, corresponding to October-November), from Greek Pyanepsia (plural), literally "the feast of cooking beans," from pyanos, name of a kind of bean, of unknown origin, + epsein "to boil, cook." At this festival a dish of pulse was offered to the god.
wasteland (n.): 1825 as one word, from waste (adj.) + land (n.). Figurative sense is attested from 1868. Eliot's poem is from 1922.
April is the cruellest month, breeding
Lilacs out of the dead land, mixing
Memory and desire, stirring
Dull roots with spring rain.
semester (n.): 1827, from German Semester "half-year course in a university," from Latin semestris, in cursus semestris "course of six months," from semestris, semenstris "of six months, lasting six months, half-yearly, semi-annual," from sex "six" (see six) + mensis "month" (see moon (n.)). Related: Semestral; semestrial.
menopause (n.): 1852 (from 1845 as a French word in English), from French ménopause, from medical Latin menopausis, from Greek men (genitive menos) "month" (see moon (n.)) + pausis "a cessation, a pause," from pauein "to cause to cease" (see pause (n.)). Earlier it was change of life.
lunacy (n.): 1540s, "condition of being a lunatic," formed in English from lunatic + -cy. Originally in reference to intermittent periods of insanity, such as were believed to be triggered by the moon's cycle. The Old English equivalent was monaðseocnes "month-sickness."
legem pone: "payment of money, cash down," 1570s, from first two words of the fifth division of Psalm cxix, which begins the psalms at Matins on the 25th of the month; consequently associated with March 25, a quarter day in the old financial calendar, when payments and debts came due.
June: sixth month, c. 1300, Iun, June, Juin, from Latin Iunius (mensis), probably a contraction of Iunonius, "sacred to Juno" (see Juno). Replaced Old English liðe se ærra "earlier mildness." Spelling variant Iune lingered until 17c.
werewolf (n.): late Old English werewulf "person with the power to turn into a wolf," from wer "man, male person" (from PIE *wi-ro- "man, freeman;" see virile) + wulf (see wolf (n.); also see here for a short discussion of the mythology). Belief in them was widespread in the Middle Ages. Similar formation in Middle Dutch weerwolf, Old High German werwolf, Swedish varulf. In the ancient Persian calendar, the eighth month (October-November) was Varkazana-, literally "(Month of the) Wolf-Men."
January (n.): late 13c., Ieneuer (early 12c. in Anglo-French), from Old North French Genever, Old French Jenvier (Modern French Janvier), from Latin Ianuarius (mensis) "(the month) of Janus," to whom the month was sacred as the beginning of the year according to later Roman reckoning (cognates: Italian Gennaio, Provençal Genovier, Spanish Enero, Portuguese Janeiro). The form was gradually Latinized by c. 1400. Replaced Old English geola se æfterra "Later Yule." In Chaucer, a type-name for an old man.
gamete (n.): "sexual protoplasmic body," 1880, coined 1878 by German cytologist Eduard Strasburger (1844-1912), the widespread attribution of the word's coinage to Mendel being apparently erroneous. From Greek gamete "a wife," gametes "a husband," from gamein "to take to wife, to marry," from PIE root *gem(e)- "to marry" (cognates: Greek gambros "son-in-law, father-in-law, brother-in-law;" Sanskrit jamih "brother, sister," jama daughter-in-law;" Avestan zama-tar "son-in-law;" Latin gener "son-in-law"). See also -gamy. The seventh month of the ancient Attic calendar (corresponding to late January and early February) was Gamelion, "Month of Marriages." Related: Gametal.
December (n.): c. 1000, from Old French decembre, from Latin December, from decem "ten" (see ten); tenth month of the old Roman calendar, which began with March.

The -ber in four Latin month names is probably from -bris, an adjectival suffix. Tucker thinks that the first five months were named for their positions in the agricultural cycle, and "after the gathering in of the crops, the months were merely numbered."
If the word contains an element related to mensis, we must assume a *decemo-membris (from *-mensris). October must then be by analogy from a false division Sep-tem-ber &c. Perhaps, however, from *de-cem(o)-mr-is, i.e. "forming the tenth part or division," from *mer- ..., while October = *octuo-mr-is. [T.G. Tucker, "Etymological Dictionary of Latin"]
instant (adj.): mid-15c., "now, present, of the moment, current," from Old French instant "near, imminent, immediate, at hand; urgent, assiduous" (14c.) and directly from Medieval Latin instantem (nominative instans), in classical Latin "present, pressing, urgent," literally "standing near," present participle of instare "to urge, to stand near, be present (to urge one's case)," from in- "in" (see in- (2)) + stare "to stand," from PIE root *stā- "to stand" (see stet).

Sense of "immediate, done or occurring at once" is from 1590s. Of processed foods, by 1912; instant coffee is from 1915. Televised sports instant replay attested by 1965. Instant messaging attested by 1994. The word was used 18c.-19c. in dating of correspondence, meaning "the current month," often abbreviated inst. Thus 16th inst. means "sixteenth of the current month" (see ultimo).
neap (adj.): Middle English, from Old English nepflod "neap flood," the tide occurring at the end of the first and third quarters of the lunar month, in which high waters are at their lowest, of unknown origin, with no known cognates (Danish niptid probably is from English). Original sense perhaps is "without power." As a noun from 1580s.
ides (n.): "middle day of a Roman month," early 14c., from Old French ides (12c.), from Latin idus (plural) "the ides," a word perhaps of Etruscan origin. In the Roman calendar the eighth day after the nones, corresponding to the 15th of March, May, July, and October; the 13th of other months. "Debts and interest were often payable on the ides" [Lewis].
September: late Old English, from Latin September (also source of Old French Septembre, Spanish Setiembre, Italian Settembre, German September), from septem "seven" (see seven). So called because it was the seventh month of the old Roman calendar, which began the year in March; Julian calendar reform (46 B.C.E.) shifted the new year back two months. For -ber suffix, see December. Replaced Old English hærfestmonað, haligmonað. Related: Septembral.
hecatomb (n.): 1590s, from Latinized form of Greek hekatombe, properly (and literally) "offering of 100 oxen," but generally "a great public sacrifice." It is a compound of hekaton "one hundred," which perhaps is dissimilated from *hem-katon, with hen, neuter of eis "one" + *katon "hundred." The second element is bous "ox" (see cow (n.)). The first month of the Attic calendar (corresponding to July-August) was Hekatombaion, in which sacrifices were made.
first (n.): 1560s, "that which is first," from first (adj.). Meaning "first day of the month" is by 1590s. In music, "instrument or voice that takes the highest or chief part of its class," 1774. From 1909 as the name of the lowest gear in an engine. In British schools colloquial use, "highest rank in an examination," 1850.
calendar (n.): c. 1200, "system of division of the year;" mid-14c. as "table showing divisions of the year;" from Old French calendier "list, register," from Latin calendarium "account book," from calendae/kalendae "calends" the first day of the Roman month -- when debts fell due and accounts were reckoned.

This is from calare "to announce solemnly, call out," as the priests did in proclaiming the new moon that marked the calends, from PIE root kele- (2) "to call, shout" (see claim (v.)). In Rome, new moons were not calculated mathematically but rather observed by the priests from the Capitol; when they saw it, they would "declare" the number of days till the nones (five or seven, depending on the month). The word was taken by the early Church for its register list of saints and their feast days.

Sometimes used in a general sense, "to insert between others" (1824). The -ar spelling in English is 17c. to differentiate it from the now obscure calender "cloth-presser."