E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A piece of meat between two slices of bread; so called from the Earl of Sandwich (the noted Jemmy Twitcher), who passed whole days in
gambling, bidding the waiter bring him for refreshment a piece of meat between two pieces of bread, which he ate without stopping from play. This contrivance was not first hit upon by the earl in the reign of George III., as the Romans were very fond of sandwiches, called by them offula.