As seen on How I Met Your Mother, used in reference to your cards in a game of blackjack, but about the bang-ability rating of a girl. In blackjack, a player says "hit" if he wants another card. Trying not to go over 21, 15 can be a bad hand, because the blackjack player isn't sure if he wants to hit or not.
"Man, did you see that girl that just walked by, she was fine"
"Ya, I guess. She was a 15"
"'Cus I am not sure if I would hit that or not"