The phrase "weak knees" means to feel very nervous or scared about something, causing your legs to feel weak or shaky.
Sentences with «weak knees»
This assisted squat form is also great for people with weak knees. (fitwirr.com)
And those with knee injuries or weak knees know all too well what difficulty a knee problem can cause. (fitnessandpower.com)
Fortunately this did not occur with yours truly at wheel, as I briefly saw an indicated 175 mph and then gave up with weak knees and moist palms. (automobilemag.com)