be/go weak at the knees

go weak at the knees

To become unsteady or flustered due to a strong emotional reaction to someone or something. Hearing the news of my father's sudden death, I went weak at the knees and could barely remain standing. Just thinking about Samantha makes me go weak at the knees.
See also: knee, weak

be/go ˌweak at the ˈknees

(informal) be/become weak because of illness, strong emotion, etc: He felt dizzy and a bit weak at the knees.Her smile made me go weak at the knees (= with nervousness, love, etc.). ▶ ˌweak-ˈkneed adj. not brave or determined
See also: knee, weak