Stacking

When a group of 2 or more people sit on each other's laps over a toilet and piss together.

(Although it is usually practised with piss, the person on the bottom of the stack could theoretically poop, as well, though it would be rude for someone higher in the stack to do so)

"Hey guys, you ever tried stacking?"
"No, but I need to go to the toilet, wanna stack with me?"

7👍 2👎