a reflex extension of the leg, caused by contraction of the quadriceps, resulting from a sharp tap on the patellar tendon; patellar reflex.
knee-jerkPronunciation: (nē'jûrk"), [key]
of or pertaining to a knee-jerk.
reacting according to a certain habitual manner; unthinking: a knee-jerk liberal.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.