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.
(1) to obtain clarity of thought or extemporaneous enlightenment, (2) to experience ease of difficulty, (3) to leave an arduous state of mind or being.
A term used when someone, usually male, is more pissed off than usual - if something crawled up your leg and bit your bollock you would quite understandably be pissed off.
Usually quite a subtle way to tell them they're acting annoyed, funny too.
Joe: " STFU DUDE!"
Jim: "Damn, what's crawled up your leg and bit your bollock ?"