Biss Share definition Flag Butt-piss. Sometimes an unintended, humorous side-effect of gay anal sex. Anal leakage. Is that shit on my bedsheets?! No. It's biss. What's that? Umm, butt piss. Eeew!! Biss by Popingay June 6, 2010 0 0 Get merch Get the Biss mug. Buy Now — $32.95 See more merch