(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. Believe me, my knees were knocking when I saw all the police cars outside of my house.

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!