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.
21👍 3👎
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"
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