weak at the knees idiom
（通常指因看到或谈到非常喜欢的人或让人害怕的事而）膝盖发软的，腿发软的
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. 吻他的念头让我膝盖发软。