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I like to read the Economist to keep my English up to date. And today, reading the following news I came across the above mentioned expression.

Merriam Webster defines it as:

reacting in a readily predictable way

I'd like to know how come such words like knee and jerk could come up with such a definition.

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    Did you ever swing your leg off the end of a chair or table and then hit it just below the kneecap?
    – Jim
    Feb 5, 2013 at 3:27
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    What @Jim said. Surely this is General Reference / common knowledge?
    – FumbleFingers
    Feb 5, 2013 at 3:33
  • Seems General Reference from etymology dictionary.
    – Lynn
    Feb 5, 2013 at 4:34
  • Yes, the reference from the etymology dictionary is self evident to anyone who understands "the figurative use appeared soon after the phrase was coined". But I suppose to "coin a phrase" is General Reference also!
    – Fortiter
    Feb 5, 2013 at 6:48
  • Should I delete the question? or perhaps get it moved to ELL.
    – Androiderson
    Feb 5, 2013 at 19:58

2 Answers 2

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The phrase is metaphoric and is derived from the standard reflexive movement of the leg when the knee is struck in a certain way. Oxford defines it as

adjective - (of a response) automatic and unthinking: a knee-jerk reaction
(of a person) responding to situations in an automatic and unthinking way: knee-jerk radicals
noun - a sudden involuntary reflex kick caused by a blow on the tendon just below the knee.

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A knee-jerk reaction is a reflex to an impact. If you have had a physical exam where the doctor checked your reflexes by tapping your knee cap with a little rubber hammer, then you've seen a true knee-jerk reaction.

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