"a union of two vowels pronounced in one syllable," late 15c., diptonge, from Late Latin diphthongus, from Greek diphthongos "having two sounds," from di- "double" (from PIE root *dwo- "two") + phthongos "sound, voice," which is related to phthengesthai "to utter a sound, sound, raise one's voice, call, talk," which Beekes reports as of "no certain etymology. None of the existing connections with semantically comparable words ... is phonetically convincing." Related: Diphthongal; diphthongization.
In uttering a proper diphthong both vowels are pronounced; the sound is not simple, but the two sounds are so blended as to be considered as forming one syllable, as in joy, noise, bound, out. An "improper" diphthong is not a diphthong at all, being merely a collocation of two or more vowels in the same syllable, of which only one is sounded, as ea in breach, eo in people, ai in rain, eau in beau. [Century Dictionary]
1782, "peasant dance popular in Italy," originally "hysterical malady characterized by extreme impulse to dance" (1630s), epidemic in Apulia and adjacent parts of southern Italy 15c.-17c., popularly attributed to (or believed to be a cure for) the bite of the tarantula. This is likely folk-etymology, however, and the names of the dance and the spider more probably share an origin in Taranto, the name of a city in southern Italy (see tarantula). Used from 1833 to mean the style of music that accompanies this dance, usually in 6/8 time, with whirling triplets and abrupt major-minor modulations. Related: Tarantism.
Those who were bitten generally fell into a state of melancholy, and appeared to be stupified, and scarcely in possession of their senses. This condition was, in many cases, united with so great a sensibility to music, that at the very first tones of their favourite melodies, they sprang up, shouting for joy, and danced on without intermission, until they sank to the ground exhausted and almost lifeless. [Babington's translation of J.F.C. Hecker, "The Epidemics of the Middle Ages," London, 1859]
a complicated word of uncertain history. First attested late 15c. in the sense "lewd person" (of either sex; since c. 1700 applied exclusively to women); probably [Middle English Compendium] from Old French baud "gay, licentious" (from Frankish *bald "bold" or some such Germanic source; see bold), despite the doubts of OED. The 15c. English word perhaps is a shortening of baude-strote "procurer or procuress of prostitutes" (c. 1300).
For the French sense evolution from "bold" to "lewd," compare Old French baudise "ardor, joy, elation, act of boldness, presumption;" baudie "elation, high spirits," fole baudie "bawdry, shamelessness." The Old French word also is the source of French baudet "donkey," in Picardy dialect "loose woman."
The second element in baude-strote would be trot "one who runs errands," or Germanic *strutt (see strut (v.)). There was an Old French baudestrote, baudetrot of the same meaning (13c.), and this may be the direct source of Middle English baude-strote. The obsolete bronstrops "procuress," frequently found in Middleton's comedies, probably is an alteration of baude-strote.
late 14c., in the Old Testament sense, from Old French jubileu "jubilee; anniversary; rejoicing" (14c., Modern French jubilé), from Late Latin iubilaeus "the jubilee year," originally an adjective, "of the jubilee," from Greek iabelaios, from iobelos, from Hebrew yobhel "jubilee," formerly "a trumpet, ram's horn," literally "ram." The original jubilee was a year of emancipation of slaves and restoration of lands, to be celebrated every 50th year (Levit. xxv:9); it was proclaimed by the sounding of a ram's horn on the Day of Atonement.
The form of the word was altered in Latin by association with unrelated Latin iubilare "to shout with joy" (for which see jubilant), and the confusion of senses has continued in the Romanic languages and English. The general sense of "season of rejoicing" is first recorded mid-15c. in English, however through early 20c. the word kept its specific association with 50th anniversaries.
As a type of African-American folk song, it is attested from 1872. The Catholic Church sense of "a period for remission of sin penalties in exchange for pilgrimages, alms, etc." was begun in 1300 by Boniface VIII.
Old English myrgð "joy, pleasure, eternal bliss, salvation" (original senses now obsolete), from Proto-Germanic *murgitha (source also of Middle Dutch merchte), noun of quality from *murgjo- (see merry; also see -th (2)). By early 13c. as "expressions or manifestations of happiness, rejoicing;" by mid-14c. as "state or feeling of merriment, jollity, hilarity." Mirthquake "entertainment that excites convulsive laughter" first attested 1928, in reference to Harold Lloyd movies.
I HAVE always preferred chearfulness to mirth. The latter, I consider as an act, the former as an habit of the mind. Mirth is short and transient, chearfulness fixed and permanent. Those are often raised into the greatest transports of mirth, who are subject to the greatest depressions of melancholy: on the contrary, chearfulness, though it does not give the mind such an exquisite gladness, prevents us from falling into any depths of sorrow. Mirth is like a flash of lightning, that breaks through a gloom of clouds, and glitters for a moment; chearfulness keeps up a kind of day-light in the mind, and fills it with a steady and perpetual serenity. [Addison, "Spectator," May 17, 1712]
"stringed musical instrument, violin," late 14c., fedele, fydyll, fidel, earlier fithele, from Old English fiðele "fiddle," which is related to Old Norse fiðla, Middle Dutch vedele, Dutch vedel, Old High German fidula, German Fiedel "a fiddle;" all of uncertain origin.
The usual suggestion, based on resemblance in sound and sense, is that it is from Medieval Latin vitula "stringed instrument" (source of Old French viole, Italian viola), which perhaps is related to Latin vitularia "celebrate joyfully," from Vitula, Roman goddess of joy and victory, who probably, like her name, originated among the Sabines [Klein, Barnhart]. Unless the Medieval Latin word is from the Germanic ones.
FIDDLE, n. An instrument to tickle human ears by friction of a horse's tail on the entrails of a cat. [Ambrose Bierce, "The Cynic's Word Book," 1906]
Fiddle has been relegated to colloquial usage by its more proper cousin, violin, a process encouraged by phraseology such as fiddlesticks (1620s), contemptuous nonsense word fiddle-de-dee (1784), and fiddle-faddle. Century Dictionary  reports that fiddle "in popular use carries with it a suggestion of contempt and ridicule." Fit as a fiddle is from 1610s.
c. 1200, "the face, countenance," especially as expressing emotion, from Anglo-French chere "the face," Old French chiere "face, countenance, look, expression," from Late Latin cara "face" (source also of Spanish cara), possibly from Greek kara "head," from PIE root *ker- (1) "horn; head." From mid-13c. as "frame of mind, state of feeling, spirit; mood, humor."
By late 14c. the meaning had extended metaphorically to "state or temper of mind as indicated by expression." This could be in a good or bad sense ("The feend ... beguiled her with treacherye, and brought her into a dreerye cheere," "Merline," c. 1500), but a positive sense, "state of gladness or joy" (probably short for good cheer), has predominated since c. 1400.
Meaning "that which makes cheerful or promotes good spirits" is from late 14c. Meaning "shout of encouragement" is first recorded 1720, perhaps nautical slang (compare earlier verbal sense, "to encourage by words or deeds," early 15c.). The antique English greeting what cheer? (mid-15c.) was picked up by Algonquian Indians of southern New England from the Puritans and spread in Indian languages as far as Canada.
"brightness, radiant energy, that which makes things visible," Old English leht (Anglian), leoht (West Saxon), "light, daylight; spiritual illumination," from Proto-Germanic *leukhtam (source also of Old Saxon lioht, Old Frisian liacht, Middle Dutch lucht, Dutch licht, Old High German lioht, German Licht, Gothic liuhaþ "light"), from PIE root *leuk- "light, brightness."
The -gh- was an Anglo-French scribal attempt to render the Germanic hard -h- sound, which has since disappeared from this word. The figurative spiritual sense was in Old English; the sense of "mental illumination" is first recorded mid-15c. Meaning "something used for igniting" is from 1680s. Meaning "a consideration which puts something in a certain view" (as in in light of) is from 1680s. Short for traffic light from 1938. Quaker use is by 1650s; New Light/Old Light in church doctrine also is from 1650s. Meaning "person eminent or conspicuous" is from 1590s. A source of joy or delight has been the light of (someone's) eyes since Old English:
Ðu eart dohtor min, minra eagna leoht [Juliana].
Phrases such as according to (one's) lights "to the best of one's natural or acquired capacities" preserve an older sense attested from 1520s. To figuratively stand in (someone's) light is from late 14c. To see the light "come into the world" is from 1680s; later as "come to full realization" (1812). The rock concert light-show is from 1966. To be out like a light "suddenly or completely unconscious" is from 1934.
Old English beste, reduced by assimilation of -t- from earlier Old English betst "of the highest quality or standing, first, in the best manner." This originally was the superlative of bōt "remedy, reparation" (Middle English bote "advantage, help, profit"), a word now surviving in its simple form only in the expression to boot (see boot (n.2)). Its comparative, better, and superlative, best, have been transferred to good (and in some cases well).
Old English bōt is from Proto-Germanic root *bat-, with comparative *batizon and superlative *batistaz. The superlative form is the source also of Old Frisian, Old Saxon, Middle Dutch best, Old High German bezzist, German best, Old Norse beztr, Gothic batists. Also in Old English as an adverb, "in the most excellent manner."
The best-laid schemes o' mice an' men
Gang aft agley,
An' lea'e us nought but grief an' pain,
For promis'd joy!
[Burns, from "To a Mouse, on Turning Her Up in Her Nest With the Plough, November, 1785"]
From late Old English as "of greatest advantage, most suitable." Best-seller as short for "best-selling book" is from 1889, apparently originally in the publishing trade; best friend was in Chaucer (late 14c.). Best girl is first attested 1881, American English; best man is 1814, originally Scottish, replacing groomsman.
fifteenth letter of the alphabet, from a character that in Phoenician was called 'ain (literally "eye") and represented "a very peculiar and to us unpronounceable guttural" [Century Dictionary]. The Greeks also lacked the sound, so when they adopted the Phoenician letters they arbitrarily changed O's value to a vowel. (Thus there is no grounds for the belief that the form of the letter represents the shape of the mouth in pronouncing it.) The Greeks later added a special character for "long" O (omega), and the original became "little o" (omicron).
In Middle English and later colloquial use, o or o' can be an abbreviation of on or of, and is still literary in some words (o'clock, Jack-o'-lantern, tam-o'-shanter, cat-o'-nine-tails, will-o'-the-wisp, etc.).
O' the common prefix in Irish surnames is from Irish ó, ua (Old Irish au, ui) "descendant."
The "connective" -o- is the usual connecting vowel in compounds taken or formed from Greek, where it often is the vowel in the stem. "[I]t is affixed, not only to terms of Greek origin, but also to those derived from Latin (Latin compounds of which would have been formed with the L. connecting or reduced thematic vowel, -i), especially when compounds are wanted with a sense that Latin composition, even if possible, would not warrant, but which would be authorized by the principles of Greek composition." [OED]
As "zero" in Arabic numerals it is attested from c. 1600, from the similarity of shape. Similarly the O blood type (1926) was originally "zero," denoting the absence of A and B agglutinogens.
As a gauge of track in model railroads, by 1905. For o as an interjection of fear, surprise, joy, etc., see oh.
The use of the colloquial or slang -o suffix in wino, ammo, combo, kiddo, the names of the Marx Brothers, etc., "is widespread in English-speaking countries but nowhere more so than in Australia" [OED].