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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.

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
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. – Androiderson Feb 5 '13 at 19:58

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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