A wishwich is a desperate attempt at a sandwich when you have a loaf of bread but the fridge is empty of everything but condiments (mayo/mustard/ketchup/relish/pickles) and you are too broke to go buy food.
"I'm so broke I ate a mustard wishwich for breakfast!"
A sandwich that is only two pieces of bread, where you wish you had something in between said pieces.
John: "Do you have anything to eat?"
Ben: " I only have bread, which means we're having some wishwiches.