The 4th Earl of Sandwich (1718--92) loved gambling. When he was playing cards he hated to leave the gaming table for meals. So he asked for a steak between two slices of bread.
Ironically, eating sandwiches at your desk now symbolises dedication to work. And most casinos do not allow you to eat snacks when seated at their gaming tables.
Alt meanings: You can sandwich something between two halves of anything. A Victorian Sandwich, for example, is a sponge cake filled with cream and jam.