your knees are 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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
your knees are knocking
you are feeling very frightened. informalFarlex Partner Idioms Dictionary © Farlex 2017
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!Farlex Partner Idioms Dictionary © Farlex 2017