An Ace is a girl who could be a 1 or an 11 on the hot scale. Like when you only see the back of a girl and she has a nice body, shes an 11, but right when she turns around, she has a God awful face and turns into a 1. Just like the Ace can be a 11 or a 1 in Blackjack
Damn dude that girl is an ace! look at that ass!" *girl turns around* "Oh gross! what is that on her face?! she totally is an ace!