knee jerk

US: (nē′jûrk′)

WordReference Random House Unabridged Dictionary of American English © 2025

knee′ jerk′, 

1. Physiology a reflex extension of the leg, caused by contraction of the quadriceps, resulting from a sharp tap on the patellar tendon;
   patellar reflex.

• 1875–80

knee-jerk  (nē′jûrk′), adj. 

1. of or pertaining to a knee-jerk.
2. [Informal.] reacting according to a certain habitual manner;
   unthinking:
   a knee-jerk liberal.

• 1895–1900

Collins Concise English Dictionary © HarperCollins Publishers::

knee jerk noun

1. Also called: patellar reflex an outward reflex kick of the lower leg caused by a sharp tap on the quadriceps tendon just below the patella

adjective kneejerk

1. made or occurring as a predictable and automatic response, without thought: kneejerk support