An expression for what happens when you are so worried that your body shakes uncontrollably. Of course, normal shaking is a normal response to everything from stress to caffeine to cold.
but knock kneed is more then that. It is where your legs are actually shaking so much that even when you attempt to stand up your knees knock against each other uncontrollably, rebounding and then crashing against each other again. This is not just something in Scooby Doo cartoons. I always believed it was an expression, myself, until I was fifteen and I got mugged.
The evidence in my life that I am an adult is that I have first hand knowledge of what both this term and "worried sick" refer to, literally.