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.

the thing that you don’t have when you’re phones percentage is over 20

When tou have the unfortunate experience where you hiccup and fart simultaneously, this is called having 'poocups'