From Wikipedia:

The first form of sandwich is attributed to the ancient Jewish sage Hillel the Elder, who is said to have put meat from the Paschal lamb and bitter herbs inside matzo (or flat bread) during Passover. The filling between the matzos served as a reminder to Israelites of their forced labor constructing Egyptian buildings. During the Middle Ages, thick slabs of coarse bread, called “trenchers”, were used as plates. After a meal, the food-soaked trencher was fed to a dog, less fortunate beggars, or eaten by the diner. Trenchers were the harbingers of open-face sandwiches.

The first written usage of the word ’sandwich’ appeared in Edward Gibbon’s journal, referring to “bits of cold meat” as a ‘Sandwich.’ It was named after the 4th Earl of Sandwich, an 18th-century English aristocrat, although he was neither the inventor nor sustainer of the food. It is said that Lord Sandwich was fond of this form of food because it allowed him to continue playing cards, particularly cribbage, while eating without getting his cards sticky from eating meat with his bare hands. An alternative theory suggests he may have spent long hours at his desk working and therefore wanted a sandwich, also to eat with his bare hands (see External links).

The Earldom refers to the English town of Sandwich in Kent — from the Old English Sandwic, meaning “sand place”.