cock·tail1 (kokÆt!lÅ), n.
1. any of various short mixed drinks, consisting typically of gin, whiskey, rum, vodka, or brandy, with different admixtures, as vermouth, fruit juices, or flavorings, usually chilled and frequently sweetened.
2. a portion of food, as seafood served with a sauce, a mixture of fruits, or juice, served as the appetizer course of a meal.
3. Pharm. a beverage or solution concocted of various drugs.
4. any eclectic mixture or miscellaneous collection.
–v.i.
5. to drink cocktails, esp. at a cocktail party: They cocktailed before going to the theater.
–adj.
6. (of women's clothing) styled for semiformal wear: a cocktail dress.
7. of, pertaining to, used in, or suitable to the serving of cocktails: cocktail onions; cocktail napkins.
[1800–10, Amer.; orig. obscure; none of numerous attempts to explain the orig. of this word or its relationship to COCKTAIL2 have won general acceptance]

cock·tail2 (kokÆt!lÅ), n.
1. a horse with a docked tail.
2. a horse that is not a thoroughbred.
3. a man of little breeding who passes for a gentleman.
[1590–1600; COCK2 + TAIL1]

Pour moi, le premier cocktail connu, c'était le grog, mais peut-être que je me trompe.

C'était un amiral anglais du XVIII°s., dont le surnom était Old Grog, qui avait imposé que le rhum servi à bord des navires de guerre soit coupé d'eau, d'une part pour rationner la consommation d'un produit coûteux, d'autre part pour conserver un peu de lucidité aux marins de Sa Majesté. Très vite, on y a ajouté du citron, pour relever le goût, et (encouragé par les médecins du bord) pour lutter contre le scorbut, fléau des longues navigations.