Region: General | Usage example: Her knee-jerk reaction was to cancel the meeting.

Knee-jerk reaction

Meaning

An immediate, automatic response.

Used when someone reacts without thinking.

Origin notes can vary by region and source; this page gives the commonly reported explanation and usage context.

Reflex that causes the leg to kick when the knee is tapped.

Medical metaphor adopted into daily speech by the mid-20th century.