Full phrase: "What crawled up your ass and died?" It is said after somebody farts and the smell is so horrible that everyone in the room runs out gagging. The implication is that the fart smells as bad as, for example, a rotting dead skunk or maggoty rat.
Todd's fart was so overpowering that Ben said, "Man, you stink! What crawled up your ass and died?"
by lyndal June 01, 2007