What does it mean to get your ass pinched in a club?

I've had a few females do it to me and honestly, where I come from I have no idea what that means. At first I turned around to see the drunk girls giggling about it felt a little awkward. It happens almost everytime I go to a club and now all I do is smile or laugh about it & then move on. A guy told me you're supposed to pinch them back but I didn't since I wasn't sure...
What does it mean to get your ass pinched in a club?
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