APAD: Knee-jerk reaction

A knee-jerk reaction is an automatic,, impulsive, and unthinking response to a situation, often characterized as overly emotional, aggressive, or predictable. Derived from the medical "patellar reflex" (where a tap below the kneecap causes a leg kick), it metaphorically describes actions taken without deliberation. It is an immediate, unthinking, or instinctive reaction to a stimulus.

The term comes from the patellar reflex, a, clinical test for spinal nerve damage (L2–L4) where a tap on the tendon causes an involuntary kick. 

It is usually used in a negative or critical context, implying a lack of thoughtful consideration.

Knee jerk describes someone who always reacts quickly and without enough thought. For example, let's say you are on a team with someone who never faces .  If brought to account, the player always blames the dropped ball on other teammates. You can say that player is having a knee-jerk reaction to criticism. They have a go-to response. It’s like a reflex they can’t control.

Now, a gut reaction also happens without thinking. But it is a bit different than a knee-jerk response. A gut reaction is a feeling. When we have a gut reaction, our instincts are telling us that something may not be right. Gut reactions are instinctual warnings.