A cocktail dating back to at least the 1860s called a John Collins was being served by the headwaiter of Limmer's Club in London, whose name was John Collins. This early version would have been served with jenever, a predecessor to gin. The name Tom Collins was to specify you wanted your drink made with Old Tom Gin. Today we generally use London Dry Gin in a Tom Collins, and a John Collins is often made with bourbon.
Later, we see the Tom Collins Hoax of 1874 in New York, where a patron at a bar would start a conversation with a stranger and ask if they knew Tom Collins. They stranger would respond that the did not know any Tom Collins. The patron would then tell a story about how they just came from such and such bar and a man named Tom Collins was tarnishing the strangers reputation. The stranger would then get up and leave the bar to go set matters straight, you would take their stool at the bar and everyone would have a good laugh at the strangers expense.
Which of these two stories is the true origin? Are they related? In any case, the Tom Collins is a delicious tall drink of gin, lemon, sugar, and soda that is one of my personal favorites, especially on a hot summer's day.