Suddenly lost consciousness and fell down, usually because of shock, fear, low blood sugar, or a medical problem.
From Old French 'faindre' (to pretend, to feign). The medical meaning evolved from the idea of the body 'pretending' to be dead, as consciousness seems to disappear temporarily.
Historically, women were taught that fainting was elegant and ladylike—Victorian society had corsets so tight they caused fainting, then called it feminine sensitivity, showing how culture literally shaped medicine and biology.
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