look up any word, like fleek:
 
1.
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
 
2.
ass that curls under
Did you check out that ladies underass?
by UnDeRcRuSt January 01, 2009
 
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
 
4.
Under the ass. When referring to the end of the year pants that girls get.
Let me sign your underass.
by me27 May 28, 2007