Gin, Lemon juice, Sugar syrup, Soda water.
It got its name from a quibble that used to be done in bars. “There’s Tom Collins out there waiting for you”.
When they went out to look for this “Tom Collins”, they could not find anyone. Meanwhile, inside, the prankster would drink the drink of the unsuspecting dupe. The prank became known as “The Great Tom Collins Hoax of 1874”