Extremely frightened to the point of being unable to move or react normally; paralyzed by fear.
This phrase emerged in the early 20th century, building on the older concept that fear could cause physical rigidity. It reflects the biological 'freeze' response to danger, one of the fight-flight-freeze reactions, where muscles tense and movement becomes difficult or impossible.
The expression captures a real physiological phenomenon - extreme fear does cause muscle tension and can literally make movement difficult. Interestingly, 'stiff' in this context predates its slang meaning for 'dead person,' though both usages reflect the same idea of rigid immobility that fear and death share.
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