knee-jerk

Meaning

1. Unthinking, not carefully considered, (nearly) automatic, spontaneous, easily predictable.
2. A sudden reflexive movement of the leg below the knee, as a reaction to a tap to the tendon just below the patella (kneecap).

Etymology / origin

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

Sources

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