Scatter-brained, reckless, or lacking good judgment; impulsive and foolish.
From 'hare' (Old English hara) + 'brain' (Old English brægen). The compound emerged in the 1500s-1600s based on folk beliefs that hares were nervous, flighty animals lacking in intelligence, making 'harebrain' equivalent to 'foolish.'
Hares were unfairly stereotyped as stupid for being nervous and easily startled, but they're actually quite intelligent—yet the slur stuck in our language, showing how a single unfair stereotype can hide inside a word for 500 years without anyone questioning it.
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