knee jerk

You experience a knee jerk when your doctor taps your knee with a rubber mallet and your leg automatically kicks out. The idiom "a knee jerk reaction" means that you respond to something in an equally unthinking way.

If you instinctively assume that someone who loves clowns wouldn't have much in common with you, that's a knee jerk, or automatic, reaction. This figurative meaning of knee jerk came from the physical reflex — which is called a "patellar reflex" by doctors, and which was discovered and named in the 1870's.

Definitions of knee jerk
1

n a reflex extension of the leg resulting from a sharp tap on the patellar tendon

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