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.