A sport in which one yells "ULTIMATE GRAB-ASS," at which point everyone in the room (or immediate area) starts to grab someone else's ass, while protecting your own ass at the same time. Sometimes it is paired with "The Floor is Lava" for extra epicness.
Dude, remember playing Ultimate Grab-Ass last night?
Oh yeah, man. I rocked that game. My ass didn't get grabbed once.