Their knees were knocking - Idioms by The Free Dictionary
(one's) knees are knocking (redirected from their knees were knocking)
(one's) knees are knocking
One is trembling or shaking with fear, anxiety, or nervousness. That poor kid's knees were knocking up there on stage while he waiting for his turn to talk. I may seem confident when I'm leading a training seminar, but my knees are knocking most of the time.
your knees are knocking you are feeling very frightened. informal
your/somebody’s ˈknees are knocking (informal) if your/somebody’s knees are knocking, they are shaking because you are nervous or afraid: It was the first time I’d ever spoken in public, and my knees were knocking!