knee-jerk

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

Origin of knee-jerk

First recorded in 1895–1900

knee jerk

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

Origin of knee jerk

First recorded in 1875–80
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words for knee-jerk

expected, predictable, reflex, spontaneous

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British Dictionary definitions for knee-jerk

knee jerk

noun
  1. Also called: patellar reflex physiol 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 thoughtkneejerk support
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for knee-jerk
n.

patellar reflex, neurological phenomenon discovered and named 1876; the figurative use appeared soon after the phrase was coined.

Online Etymology Dictionary, © 2010 Douglas Harper

knee-jerk in Medicine

knee jerk

n.
  1. patellar reflex
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.