knee-buckler

A curveball (or occasionally a knuckleball) that fools the hitter, causing his or her knees to buckle (i.e. bend).

Anything so stunning as to have a weakening effect on a person.

This guy is a sensitive and gentle, yet exciting sex partner, certainly not rough but always leading the dance. She could get weak, just anticipating him, a real knee buckler.

“Hi, Jimmy!” His friends stared back and forth between him and myself with their mouths open. Bella smiled a patented knee buckler at him.