Air Passing Over Poop. Farting while you have a turd in the chamber.
Yuck! Who let out the APOP?
means the Amazing Power of Pussy. Used to describe a situation in which a man show outrageous, unthinkable behavior in pursuit of a woman. This would include moving to Alaska in February, or abandoning family/state/governorship to fly out of the country to be with an Argentine mistress. Often associated with disastrous or catastrophic consequences.
Dude #1: Hey, I heard Jason is moving to Juneau. He hates cold weather! What's up with that?!
Dude #2: Yeah, but he's crazy about some chick that wants him to move up there.
Dude #1: That's APOP for you.