A person who is scatter-brained, thoughtless, or impulsive; someone who acts without thinking.
Compound of 'hair' and 'brain,' where 'hair-brained' is the more standard spelling. The exact origin is debated, but it may originally have meant 'brain as thin as a hair' or referred to someone with scattered thoughts.
The phrase 'hare-brained' (meaning a crazy hare) and 'hair-brained' (thin brain) might be confused origins, showing how English words can blend together over centuries.
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