Shamelessly stolen from the OED online...
gay, a., adv., and n.
A. adj.
1. a. Of persons, their attributes and actions: Full of or disposed to joy and mirth; manifesting or characterized by joyous mirth; light-hearted, exuberantly cheerful, sportive, merry.
b. Of a horse: Lively, prancing. [So in Fr.] rare1.
c. With implied sense of depreciation: Airy, off-hand.
d. In poetry: Applied to women, as a conventional epithet of praise. Obs. (Cf. FREE a. 3.)
e. the gay science: a rendering of gai saber, the Provençal name for the art of poetry.
f. Forward, impertinent, too free in conduct, over-familiar; usu. in phr. to get gay. U.S. slang.
g. Of a dog's tail: carried high or erect.
2. a. Addicted to social pleasures and dissipations. Often euphemistically: Of loose or immoral life. Esp. in gay dog, a man given to revelling or self-indulgence; gay Lothario: see LOTHARIO.
b. Hence, in slang use, of a woman: Leading an immoral life, living by prostitution.
c. Of a person: homosexual. Of a place: frequented by homosexuals. slang.
3. Bright or lively-looking, esp. in colour; brilliant, showy.
4. Finely or showily dressed. Now rare.
5. In immaterial sense: Brilliant, attractive, charming. Formerly also of reasonings, etc.: Specious, plausible.
6. a. Brilliantly good; excellent, fine. Obs.
b. ironically. Obs.
c. to have a gay mind: ‘to have a good mind’, to be very much inclined.
7. Of quantity or amount. Pretty good, ‘tolerable’, ‘middling’. Sc. and north. Also GEY.
8. dial. In good health; well, convalescent.
9. Comb., chiefly parasynthetic, as gay-coloured, -flowered, -hearted, -humoured, -looking, -seeming adjs.; gay cat U.S. slang, a young or inexperienced tramp; a hobo who accepts occasional work; (see also sense 2c); gay deceiver, (a) a deceitful rake (RAKE n.5); (b) pl. slang = FALSIES n. pl.; gay-feather U.S., the name of a plant (see quot.); Gay Gordons, (a) (see quot. 1925); (b) a Scottish dance popular in old-time and modern dancing; Gay Liberation (Front) orig. U.S., (a movement for) the liberation of homosexuals from social stigma and discrimination; also with lower-case initials, and abbrev. as Gay Lib (see LIB).
B. adv.
1. a. Brightly, showily = GAILY 1. b. In a gay mood, joyously = GAILY 2. Obs.
2. Very. Also in weaker sense: Considerably, ‘pretty’. Frequent in dial. a gay few = a good few: see FEW 2d. Often written GEY.
3. Comb. Chiefly with pres. and pa. pples., as gay-beseen (see BESEE II), -careering, -chirping, -motleyed, -painted, -shifting, -smiling, -spent, -spotted, -throned.
C. n. [the adj. used absol.]
1. A gay lady. Also, rarely of a man, a ‘gallant’. Obs.
2. a. Anything that looks gay or showy; an ornament; esp. one that is used to amuse a child. Now dial.
b. fig. A ‘toy’, childish amusement. Obs.
3. A picture in a book. Now dial. (chiefly used by children).
4. slang (orig. U.S.). A homosexual, esp. a male homosexual. Cf. A 2c above.
gay, v.
a. intr. To be gay. b. trans. To make gay; to give a bright and pleasant look to; to embellish. Usu. with up. Hence gayed ppl. a.
...good lord! Doesn't that cover everything?
― , Tuesday, 14 January 2003 04:06 (twenty-three years ago)
eight years pass...