Why was "tantrum" trending last week?
knee-jerk reflex n.
See patellar reflex.
A sudden involuntary forward movement of the lower leg that can be produced by a firm tap to the tendon located just below the kneecap.
Note: The term is loosely applied to any response or belief that is automatic rather than thoughtful. In this sense, it is usually a term of reproach: “Smith is a knee-jerk liberal.”