1. Literally expelling urine from one's ass after having the urine injected into the anus during the act of a pistol star.
2. A specific type of diarrhea, wherein the contents of one's ass is expelled in a liquified, piss-like manner.
Paul was buttpissing all night after the girls all gave him a good pistol star.
Grant had to seriously buttpiss the morning after Joey gave him a pistol star for his birthday.
Diarrhea, or liquid shit that comes out of your ass. Sometimes occurs when eating Mexican food, eggs, or drinking too much whole milk. Akin to shitting a lot of Yoo-Hoo.
Holy fuckin shit dude, your mom's egg casserole gave me some SERIOUS buttpiss!!