knee-buckler

noun2

1. •
   (slang) A curveball (or occasionally a knuckleball) that fools the hitter, causing his or her knees to buckle (i.e. bend).
2. •
   (slang) 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. — 2008