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Refers to the very short skirts that girls wear, which shows the lower section of their buttocks.
Dude, look at that girls underass! I'm sure I can see gash.
by hamstar... March 27, 2010
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3
Hind quarters visible due to a female wearing only a shirt, with no undergarments.
Did you see the underass in Lindsay's profile picture?

Underass is far better than underboob in my honest opinion.
by FIDDLEmeTimbers April 03, 2010
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