Most people look up in the dictionary only those words they don't know, which is a shame because there is some interesting information hidden among common entries. Here's a gem of a definition of cocktail from the OED (the OED is a mammoth tome, and although the editors are constantly updating entries, there are plenty of words in the Dictionary whose definitions haven't been touched since the 19th century, cocktail being one of them):
Cocktail: ‘A person assuming the position of a gentleman, but deficient in thorough gentlemanly breeding.’
This meaning was in use in the mid-19th century. It came about as a result of a horse-racing term broadening in application:
Cocktail: ‘Any horse of racing stamp and qualities, but decidedly not thorough-bred, from a known stain in his parentage’ ( this is taken from the 1870 Dictionary of Rural Sports).
And this meaning came about because one of the earliest meanings of cocktail was a cock-tailed horse (ie one with a docked tail), and it was generally believed that cock-tailed horses were not thoroughbreds.
Another meaning of cocktail that was in use at the same time as the above horse-related meaning (early 19th century) was the meaning that is still common today ie a mixed alcoholic drink. The OED says that the reason why the word came to be applied to alcoholic drinks is unknown. The first citation for the drink meaning is a few years earlier than the first citation for the horsey meaning.