Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • adj. Slang Easily predictable; automatic: "quick, easy laughs and knee-jerk responses” ( New York).
  • adj. Slang Reacting spontaneously in the expected manner: a knee-jerk cynic.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • adj. Automatic, spontaneous, easily predictable
  • n. A sudden reflexive movement of the leg below the knee, as a reaction to a tap to the tendon just below the patella (kneecap).

from The Century Dictionary and Cyclopedia

  • n. A sudden jerking of the knee, caused by a contraction of the quadriceps femoris, evoked by a blow on the patellar tendon or in any way that gives the quadriceps a sudden tug. Also called patellar tendon reflex and knee-kick.

Etymologies

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

Examples

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