Top Definition
The excuse you should ALWAYS give to your girlfriend when she bitches to you about grabbing/slapping her ass.
Zach: (slowly reaches for a game of grab-ass)

Kim: What the fuck, Zach! We're in public!

Zach: Wha...WHAT!?!? That wasn't me! That was The Underpants Monster!!!!

Kim: Oh, I'm sorry for yelling.

Zach: Damn right you are! (slaps Kim's ass)
by AKKKHMED May 30, 2009
5 Words related to Underpants Monster

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