A bitter yellow-green fluid produced by the liver to aid digestion, or metaphorically, bitter anger or irritability.
From Latin bilis, related to ancient Greek chole. Medieval medicine considered bile one of four bodily humors that determined temperament and health.
Ancient physicians believed excess bile caused anger and irritability, which is why we still say someone is 'bilious' when they're grumpy! This shows how medical theories can permanently influence our language.
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