Agressive diarrhea, more violent in nature than regular diarrhea, and where each episode requires a longer than normal visit to the restroom. Similar in nature to extreme vomiting after drinking WAY too much alcohol after a long night of partying except that material is ejected from the other end. (The word is funnier if pronounced "Buttheavin' ".)
Dude, why were you in there so long?
Sorry, I was butt heaving.
Don't go in there yet, someone was just in there buttheavin'.