up to Idioms

Knee jerk reaction

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Generated image of a woman getting a knee reflex test
A knee jerk reaction is a quick response made without thinking carefully. This idiom is used to describe when someone reacts straight away without thinking.

The expression comes from a simple medical reflex. When a doctor taps the knee just below the kneecap, the leg automatically kicks forward without you meaning to.

His first response was a knee jerk reaction, but later he realized he needed to think about the problem more carefully.

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