(one's) knees are shaking(redirected from their knees were shaking)
(one's) knees are shaking
One is trembling or shaking with fear, anxiety, or nervousness. That poor kid's knees were shaking 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 shaking most of the time.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.