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weak at the knees

Definition

If you go weak at the knees, you lose your strength and feel you are going to fall over, usually because of seeing or talking about either someone you are very attracted to or because of something unpleasant or frightening: The thought of kissing him made me go weak at the knees.Weakness and vulnerability
(Definition of weak at the knees from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)