Another name for bad diarrhea. Especially if there is a lot of it and it comes out with a lot of force, causing you to bend over and strain a lot, almost like you are dry-heaving or about to barf. But you do it out your butt instead of your mouth. Butt barfing.
Man, what is up? I heard you in the bathroom and it sounded like a waterfall in the toilet?
Uhhhhh, sorry dude... I had major butt barf. Might not want to go in there for a while.