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Knee jerk reaction
This is something with an automatic response. Its origin comes from the
reaction one's knee gets when sharpely hit.
This sent in from a visitor:
The knee jerk is, in anatomy/physiology, one of the "reflex"
reactions. The nerve signal travels from the point struck just below the
kneecap to the spine and triggers the muscle response. The brain is not
involved. Thus the main thrust of the idiom as used to denote an "unthinking"
reaction, with the implied insult.
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