when you have a shit and it is basically water coming out of your ass
'john did a hell of a bum piss'
by Mark9288 January 23, 2007
bum piss is when you have had an illness or it can happen when you take to many drugs.
your guts empty them selves like passing water through your bottom = bum piss
by peter mills August 24, 2006
The event of an extremley runny shit, usually caused by recent diets.
I ate a whole box of bran flakes and Iv'e been bumpissin' for hours!
by o0 I3LACKOUT Oo August 03, 2008
The act of sending BUM URINE to someone via the mail
"damm dude you send you boss BUMPISS!" he is an asshole
by chubbyshawn November 16, 2007