A sandwich comprised of ones last few remaining sandwich materials to take the place of a whole meal. A lastwich is usually overly stuffed and full of the strangest things. The term lastwich only applies when you have no sandwich materials left after the lastwich is made.
John: Dude, I had nothing in the freezer so I comprised a lastwich out of the rest of my lunchmeat ham, eggs and pickles.
Joe: Fuckin' gross, dude.