When two men (the bread) catch an unsuspecting victim (the filling) in between them and begin to squeeze. For the victim it's like being caught in a two way bear hug.
Jimmy: "Dude, Daniel had two of his ribs broken when Joe and Tim caught him in a Man Sandwich."
John: "Those two always hand out one brutal Man Sandwich."
Man Sandwich (N.) When two men Squeeze themselves together against a woman making them a man sandwich. It is possible that you can have a double decker, but that would have to include 3 men and 2 women.
When I got home from the bar, I stumbled into the kitchen, grabbed some bread and made a lazy man sandwich. It had pickles and mayonnaise in it -- and that's it.