1762, said to be a reference to John Montagu (1718-1792), 4th Earl of Sandwich, who was said to be an inveterate gambler who ate slices of cold meat between bread at the gaming table during marathon sessions rather than rising for a proper meal (this account of the origin dates to 1770).
It also was in his honor that Cook named the Hawaiian islands (1778) when Montagu was first lord of the Admiralty (hence the occasional 19c. British Sandwicher for "a Hawaiian"). The family name is from the place in Kent, one of the Cinque Ports, Old English Sandwicæ, literally "sandy harbor (or trading center)." For pronunciation, see cabbage. Sandwich board, one before and one behind the carrier, is from 1864.
"insert between two other things," 1841, from sandwich (n.), on the image of meat pressed between identical pieces of bread. Related: Sandwiched; sandwiching.
also B.L.T., type of sandwich, initialism for bacon, lettuce, and tomato, the ingredients; 1940s, American English.
1854, "molten metal, a substance in a melted condition," from melt (v.). In reference to a type of sandwich (typically tuna) topped by melted cheese, by 1956, American English.
by 1974, originally in an Italian context, where the word means "small bread rolls," typically filled sandwich style; plural of panino, a diminutive of pane "bread," from Latin panis "bread," from PIE root *pa- "to feed." Used since c. 1980 on U.S. restaurant menus in reference to sandwiches made with a small flat loaf of Italian bread.
Today they have a well-established niche in any sandwich outlet. In the process of their Anglicization, their plurality has been a problem; increasingly panini is interpreted as singular, and a new plural paninis has been created .... [Ayto, "Diner's Dictionary"]