nine from behind
A girl who you can't see from the front, or maybe isn't good looking from the front, but looks damn good from behind. The nine, of course, refers to the 2nd-highest rating on the standard scale from 1-10.
Guy #1: Damn, look at the ass on that brunette!
Guy #2: Careful, she could be a nine from behind.
Guy #2: Careful, she could be a nine from behind.