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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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closed as general reference by FumbleFingers, Jim, Robusto, Lynn, tchrist Feb 5 '13 at 12:14

This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information.If this question can be reworded to fit the rules in the help center, please edit the question.

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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 '13 at 3:27
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What @Jim said. Surely this is General Reference / common knowledge? –  FumbleFingers Feb 5 '13 at 3:33
    
Seems General Reference from etymology dictionary. –  Lynn Feb 5 '13 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 '13 at 6:48
    
Should I delete the question? or perhaps get it moved to ELL. –  Exception Al Feb 5 '13 at 19:58

2 Answers 2

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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