1) When Ian pisses in his hand and then rubs it all over your gooch
2) eating the fuckin urinal cake
Related to the adjective "possin." Pissum is an insult, used on pretty much anyone, but its meaning is 'whores that piss on the streets for fun.'
"Joey is literally a pissum. Do you see what he does at night?"
"Damn all these pissums at this party!"
"Winry, I've gotta divorce you. You're just such a pissum."
Pissume: when someone acts unaware that they are taking the piss out of you.
Rob: I think you pissume that I believe, you believe what you are saying!
Karen: Nah… you're assuming I’m taking the piss... I’m serious!