[1] : 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.

[2] : 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

[3] : WordNet (r) 2.0

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

Knee Jerk >> K Words
Knee Jerk, definition of term: Knee Jerk
knee+jerk_pag1.html

Jerk >> J Words
Jerk, definition of term: Jerk
jerk_pag1.html

Knee >> K Words
Knee, definition of term: Knee
knee_pag1.html

Jerk Off >> J Words
Jerk Off, definition of term: Jerk Off
jerk+off_pag1.html

Knee Joint >> K Words
Knee Joint, definition of term: Knee Joint
knee+joint_pag1.html

Wounded Knee >> W Words
Wounded Knee, definition of term: Wounded Knee
wounded+knee_pag1.html