Knee-jerk reaction

Meaning

An immediate, automatic response.

Usage

Used when someone reacts without thinking.

Origin & History

Origins and dates here are approximate summaries. Many idioms have disputed or unknown origins, and not every date has been checked against a historical dictionary, so treat specific dates and stories as a starting point rather than a citation.

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

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