knee jerk

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A reflex contraction of the quadriceps muscle resulting in a sudden involuntary extension of the leg, produced by a sharp tap to the tendon below the patella; patellar reflex.

from the GNU version of the Collaborative International Dictionary of English.

• (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.
• noun emotional and predictable; -- of certain people and their reactions to events.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Alternative spelling of knee-jerk.
• noun Alternative spelling of knee-jerk.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a reflex extension of the leg resulting from a sharp tap on the patellar tendon