The Collaborative International Dictionary of English v.0.48:

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.]

The Collaborative International Dictionary of English v.0.48:

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

WordNet (r) 3.0 (2006):

knee jerk
    n 1: a reflex extension of the leg resulting from a sharp tap on
         the patellar tendon [syn: knee jerk, knee-jerk reflex,
         patellar reflex]