Knee-jerk definitions

A reflex contraction of the quadriceps muscle resulting in a sudden involuntary extension of the leg, produced by a sharp tap to the tendon below the patella; patellar reflex.
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Easily predictable; automatic.
adjective
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Reacting spontaneously in the expected manner.

A knee-jerk cynic.

adjective
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noun
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Designating, characterized by, or reacting with a response regarded as automatic and predictable.
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A reflex contraction of the quadriceps muscle resulting in a sudden involuntary extension of the leg, produced by a sharp tap to the tendon below the patella; patellar reflex.
noun
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Alternative spelling of knee-jerk.
adjective
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Alternative spelling of knee-jerk.
noun
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He gave a knee-jerk response.

adjective
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A sudden reflexive movement of the leg below the knee, as a reaction to a tap to the tendon just below the patella (kneecap).
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Origin of knee-jerk

knee +‎ jerk, First attested 1876, obviously derived from a figurative sense of the patellar reflex.