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• knee jerk
  knee jerk

  noun

  a reflex extension of the leg, caused by contraction of the quadriceps, resulting from a sharp tap on the patellar tendon; patellar reflex.
• knee-jerk
  knee-jerk

  adjective

  of or relating to a knee-jerk.

knee jerk

¹ American  

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.

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knee-jerk ² American  

[nee-jurk] / ˈniˌdʒɜrk /

adjective

1. of or relating to a knee-jerk.

2. Informal. reacting according to a certain habitual manner; unthinking.

   a knee-jerk liberal.

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knee jerk British  

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

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

   kneejerk support

Etymology

Origin of knee jerk¹

First recorded in 1875–80

Origin of knee-jerk²

First recorded in 1895–1900

Example Sentences

“Investors do not want to see this and the negative knee jerk reaction is being seen,” Ives said of Alphabet stock’s drop.

From Barron's • Jun. 22, 2026

"Everyone recognizes there is a growing antibiotic resistance crisis, and the knee jerk reaction is to develop new antibiotics," You added.

From Science Daily • Feb. 22, 2024

Unite's deputy regional secretary, Davy Thompson, said minimum service levels are "the most invidious example of knee jerk legislating this country has seen in a long time".

From BBC • Jan. 13, 2024

“A sensible conversation is needed when balancing community safety with personal liberty. This is not a time for knee jerk responses,” he said.

From Seattle Times • Dec. 6, 2023

Cross the knee to be stimulated over the other leg, and tap the tendon just below the knee cap, and the knee jerk appears.

From Psychology A Study Of Mental Life by Woodworth, Robert S.