Top Definition
1. When a female wears really short shorts and reveals the underside of her ass to passers by.

2. God's gift to ass men.

3. Also known as ass cleavage.
Adam: holy shit that chicks ass is hanging out! I think my dick just moved.

Craig: That my friend is some grade A UnderBum.
by Hector the molester July 28, 2010
When a female has a second bum
almost like a double chin

"does my underbum look big in this?"

"does this skirt emphasize my underbum?

"underbum is way rank"
by Chickenpastababy October 13, 2011
An area located beneath the Buttocks, If ticklish it is open to the act of Under-Bumming, where a second party uses a hand to squeeze the area as a method of acute torture.

Usually not to extract information, But to extract uncontrollable laughter and strange noises from the Under-Bumee.
Person 1 : I'm bored, I'm going to Under-bum you!

Person 2 : NO! I cant take another Under-bumming!
by Thomasjeffy December 09, 2014
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