knee jerk

Short definition from Wiktionary

Adjective

1. 1
   Alternative form of knee-jerk.
2. 2
   Unthinking, not carefully considered, (nearly) automatic, spontaneous, easily predictable.

Noun

1. 1
   Alternative form of knee-jerk.
2. 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).