It's a noun, a verb describing the action, and an exclamation. A poo cup is the act of farting in your cupped hand, then quickly holding it over an unsuspecting friend or co-worker's nose and mouth. You must shout "poo cup" as you make contact, which startles your victim, causing them to breathe in deeply.
by John Sag July 26, 2006
by WADEC January 19, 2010
When the poo juice seeps out of the asshole after hardcore anal, and slowly slides down the thigh, then the partner begins to suck it off, referring to the "suction cup"
by Sinclair o'neil November 3, 2012