Word Study
knee jerk
CIDE DICTIONARY
knee jerk,
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.]
knee jerk, n.
emotional and predictable; -- of certain people and their reactions to events. [PJC]
For further exploring for "knee jerk" in Webster Dictionary Online