Foolish, silly, or reckless; showing poor judgment and lacking careful thought.
From 'hare' (the animal) + 'brained,' dating to the 1500s. People observed that hares were jumpy and easily startled, leading to the belief they had small or scattered brains, making the phrase mean 'having a mind like a nervous rabbit's.'
This insult accidentally reveals medieval and Renaissance misunderstandings of animal anatomy—hares actually have perfectly normal brains for their size, but because they move erratically when scared, people assumed their minds were as scattered as their running patterns.
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