The Tom Collins is from the genesis era of cocktails. Its origin lies in the heady days of the early 1800s, though historians still disagree over which side of the Atlantic it was born on. If it is American, it was named after the Tom Collins Hoax of 1874, a contagious joke in which New Yorkers would ask after a fictitious Tom Collins, claiming that he had been saying things about you.
[short excerpt from Cocktails of the Movies]
2 oz / 50 ml London gin
1 oz / 25 ml freshly squeezed lemon juice
1⁄2 oz / 12.5 ml sugar syrup