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.
YOU: "POO CUP!" VICTIM: "Ahh, damn man that's nasty!"
by John Sag July 25, 2006