Informal designating, characterized by, or reacting with an automatic, predictable response
See knee jerk in American Heritage Dictionary 4
noun
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.
(nēˈjûrkˌ)
adjective Slang
Easily predictable; automatic: “quick, easy laughs and knee-jerk responses”(New York).
Reacting spontaneously in the expected manner: a knee-jerk cynic.