If you take the American side of the story, the Tom Collins evolved from the “The Great Tom Collins Hoax” of 1874, in which pranksters would tell a friend they had run into one “Tom Collins” at a bar around the corner who had said some slanderous things about said friend. Said friend would then leave to find “Tom Collins” at the bar around the corner, sparking a goose chase of, perhaps, not-so-epic proportions.
Or you can choose the British side, which (more likely) suggests that the Tom Collins was the creation of London bartender John Collins, who dreamed up an eponymous gin punch in the latter half of the 19th century, that, when made with Old Tom Gin, presumably became the Tom Collins. The first published recipe appears in Jerry Thomas’s 1876 The Bartender’s Guide. By either route (or perhaps a combination of the two), the Tom Collins is a spritzy drink made of lemon, sugar, soda water and gin—which combine to form, what is essentially, the original hard lemonade.