This is when a girl is getting fucked up the ass so hard that both her and the person fucking her knees buckle from the forces of fucking and getting fucked. Best observed in any porno where anal poundings are involved.
Dude me and my girl were fucking, it was Knee Buckling Butt Fucking night, she was so tired after that she fell asleep with my cock in her ass, what a night!

Knee Buckling Ass Fucking is the way to go
by Overloard October 08, 2010

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