Etymology
Advertisement
month (n.)

"one-twelfth part of a year; one of the twelve parts into which the calendar year is arbitrarily divided," Old English monað, from Proto-Germanic *menoth- (source also of Old Saxon manoth, Old Frisian monath, Middle Dutch manet, Dutch maand, Old High German manod, German Monat, Old Norse manaðr, Gothic menoþs "month"), which is related to *menon- "moon" (see moon (n.)). Originally the month was the interval between one new moon and the next (a sense attested from late Old English).

Its cognates mean only "month" in the Romance languages, but in Germanic they generally continue to do double duty. The development of the calendrical meaning for words from this root in Greek (mēn) and Latin (mensis) was accompanied by the creation of new words for "moon" (selēnē, luna). The phrase a month of Sundays "a very long time" is from 1832 (roughly 7 and a half months but never used literally).

Related entries & more 
Advertisement
twelve-month (n.)
"a year," Old English twelf-monð; see twelve + month.
Related entries & more 
monthly (adv.)

"once a month, in every month," 1530s, from month + -ly (2).

Related entries & more 
monthly (adj.)

1570s, "pertaining to or relating to a month," from month + -ly (1). Old English had monaþlic, but the modern words seem to be separate formations. From 1580s as "continued for a month;" 1640s as "done or happening once every month." As a noun, "a magazine or periodical published once a month," by 1833.

Related entries & more 
*me- (2)
*mē-, Proto-Indo-European root meaning "to measure." Some words may belong instead to root *med- "to take appropriate measures."

It forms all or part of: amenorrhea; centimeter; commensurate; diameter; dimension; gematria; geometry; immense; isometric; meal (n.1) "food, time for eating;" measure; menarche; meniscus; menopause; menses; menstrual; menstruate; mensural; meter (n.1) "poetic measure;" meter (n.2) unit of length; meter (n.3) "device for measuring;" -meter; Metis; metric; metrical; metronome; -metry; Monday; month; moon; parameter; pentameter; perimeter; piecemeal; semester; symmetry; thermometer; trigonometry; trimester.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mati "measures," matra "measure;" Avestan, Old Persian ma- "to measure;" Greek metron "measure," metra "lot, portion;" Latin metri "to measure."
Related entries & more 
Advertisement
menstrual (adj.)

late 14c., "pertaining to menses of females," from Old French menstruel and directly 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.)). Occasionally, in astronomy, "monthly" (1590s).

Related entries & more 
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).
Related entries & more 
calends (n.)
c. 1200, "a day as reckoned back from the first of the following month" (as fourteenth calend of March = February 16th), from Latin kalendae "first day of the month" in the Roman calendar (see calendar). From mid-14c. as "the first day of the month;" late 14c. as the beginning of anything.
Related entries & more 
menstruous (adj.)

"having the monthly flow or discharge," early 15c., from Old French menstrueus and directly from Medieval Latin *menstruosus, from Latin menstruum "of or belonging to menstruation," from menstruus (adj.) "that happens every month, monthly," from mensis "month" (see moon (n.)).

Related entries & more 
August 
eighth month, late 11c., 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.), and compare Augustus). One of two months given new names to honor Roman leaders (July being the other), the Romans also gave new imperial names to September (Germanicus) and October (Domitian) but these did not stick.

In England, the name replaced native Weodmonað "weed month." Traditionally the first month of autumn in Great Britain, the last of summer in the U.S.
Related entries & more