I'm going to be completely serious here
It doesn't mean anything. It depends on the woman really. Have you ever seen her slap anyone else on the ass? Do you hang around her enough to be able to make that kind of observation? If you're the only one, ask her if she likes you. It can't hurt. If you ask her how she feels without revealing your feelings, then you're in a win-win situation. If she likes you, you can ask her out (up to you at this point). If she doesn't like you, then you were just clearing up some confusion and you remain friends.
If she tends to slap asses, then don't bother reading into it at all. Just enjoy having her hand on your ass