The sandwich is named after John Montagu, 4th Earl of Sandwich, an eighteenth-century English aristocrat. It is commonly said that Lord Sandwich, during long sessions of cribbage and other card games at public gambling houses, would order his valet to bring him salt beef between two pieces of toasted bread. He was fond of this form of food because it allowed him to continue gambling while eating, without the need for a fork, and without getting his cards greasy from eating meat with his bare hands. The dish then grew in popularity in London, and Sandwich's name became associated with it.

Source: Wikipedia