From Longman Dictionary of Contemporary Englishweak at the kneesweak at the kneesWEAKfeeling strange because of strong emotions His smile made her go weak at the knees. → weak
Examples from the Corpus
weak at the knees• A quick bit of mental arithmetic was enough to make Chrissy weak at the knees.• Now we know the real meaning of going weak at the knees.• Or we would shower in our bathroom, whose tiles and design would make Martha Stewart weak at the knees.• Pain and nausea swept over him in waves that left him hot and sticky and weak at the knees.• The idea makes me feel weak at the knees.• Instead, here she was, going weak at the knees like an adolescent schoolgirl. 