If you were stranded on a desert island, what would you eat? .........The sand which is there.
As B00 has put so eloquently, there is a story that he was an enthusiastic gambler and when he didn't wish to leave the gaming table - or possibly just his desk - to go to dinner he would ask for someone to bring him some meat between a couple of slices of bread. Later, when others began asking for the same thing, they would say something like, "I'll have what Sandwich is having..." Allegedly it caught on, and people just started calling for "a Sandwich" when they wanted meat between two slices of bread. The word sandwich is therefore an eponym - a word or phrase whose origin is a person or historical figure. Another example is the Achilles Heel, or the Adam�s Apple. Similarly to the above, The Sandwich Islands are also named after John Montagu (4th Earl of Sandwich) as was the patron to James Cooke who ventured to the Islands.
By most accounts John Montagu didn't shrink from enjoying life. It's said that he was a corrupt, devil-worshiping sex fanatic who enjoyed gambling. My kinda guy!