One reference states the term ‘Cocktail’ came from bartenders keeping and mixing the tails of the liquor, the spilled booze from pouring drinks, and selling these mixed ‘tails’’ at the bargain ‘cocktail’ price. I remember at the bar calling this horrid concoction the mongoose!
A second idea is that the French inventor, Antoine Amédée Peychaud, while in New Orleans, would drink his brandy and bitters in an egg cup called a coquetier, pronounced cocktay. (You’ve seen these on British television shows.) This interesting nuance took notice, and before long, any mixed alcohol drink in a fancy drinking vessel was called a cocktail.
A third story, and the one I like best, says that at the races, horses that weren’t thoroughbreds had docked tails. These docked tailed horses were a mixture of other breeds, just like the alcohol in the glass not being in it’s pure form but mixed. To all those sauced race goers, the fun new term at the track bar made sense and took a foot-hold. ‘Docked-Tails’ became ‘Cocktails’ and that’s all she wrote!