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
1875-80

knee-jerk

[nee-jurk] /ˈniˌdʒɜrk/
adjective
1.
of or pertaining to a knee-jerk.
2.
Informal. reacting according to a certain habitual manner; unthinking:
a knee-jerk liberal.
Origin
1895-1900
British Dictionary definitions for knee-jerk

knee jerk

noun
1.
(physiol) 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
modifier
2.
made or occurring as a predictable and automatic response, without thought: kneejerk support
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.

knee-jerk in Medicine

knee jerk n.
See patellar reflex.

Slang definitions & phrases for knee-jerk

knee-jerk

modifier

: one more gesture to the knee-jerk hawks in the Congress/ Yet McDonald is no mere knee-jerk critic of the evangelicals

noun
  1. A reflexlike action or response: Being nasty to women is a knee-jerk with him (1958+)
  2. A person who reacts with a reflexlike response: a seventy-year old knee-jerk best remembered for castigating the Reverend Bill Moyers for dancing the frug in the White House (1958+)
verb

: We need less kneejerking on both sides in these arguments about the environment (1980s+)

[fr the patellar reflex, described as knee jerk by 1876]