Definition of Knee-jerk

Knee-jerk: Adjective meaning readily predictable to the point of being automatic. Knee-jerk often has a negative connotation and conveys the idea of an all-too-hasty, impulsive, irrational response based on a preset idea. For example, a dictator's knee-jerk response to a democratic movement is to suppress it. The term is derived from the reaction which has been so often tested as to become a familiar figure of speech.

For more about the knee-jerk reaction, see: Knee jerk.