From Longman Dictionary of Contemporary Englishknees knocking (together)knees knocking (together)FRIGHTENEDCOLDif your knees are knocking, you are feeling very afraid or very cold → knee
Examples from the Corpus
knees knocking (together)• Years later, Ogwen apprenticeship having been served, we began to aspire to their routes, knees knocking at our effrontery. 