knock knees together
knock (one's) knees together
To shake or tremble uncontrollably, as from cold, fear, or anxiety. Usually used in the continuous tense. You could tell the poor kid was knocking his knees together as he stood up there on the stage. I may seem confident when I'm leading one of these training seminars, but I'm usually knocking my knees together the whole time. We stood outside the mall knocking our knees together in the frigid cold as we waited for the bus to arrive.
knock one's knees together
Fig. [for one's knees] to shake together from fright. I stood there freezing for ten minutes, knocking my knees together in the cold. It takes a lot of energy to knock your knees together.