lag ass
When you're trying to fuck someone in the ass, and their ass is moving like it's got its own brain and won't match your rhythm.
I was trying to fuck her, but her ass was doing its own thing like it had a life of its own.
He was trying to fuck me, but my ass was like, 'Nope, not today.'
They tried to sync up, but her ass was lagging like it had a bad connection.