'Knee jerk' definitions:

Definition of 'knee jerk'

From: WordNet
noun
A reflex extension of the leg resulting from a sharp tap on the patellar tendon [syn: knee jerk, knee-jerk reflex, patellar reflex]

Definition of 'Knee jerk'

From: GCIDE
  • Knee jerk \Knee jerk\ (Physiol.) A jerk or forward kick of the lower part of the leg produced by a blow or sudden strain upon the patellar tendon of the knee, which causes a sudden contraction of the quadriceps muscle; the patellar reflex. [Webster 1913 Suppl.]

Definition of 'Knee jerk'

From: GCIDE
  • Knee jerk \Knee jerk\, n. emotional and predictable; -- of certain people and their reactions to events. [PJC]