In a reckless, foolish, or impulsive manner; without careful thought or judgment.
From 'harebrained' (hare + brain + -ed adjective suffix) + '-ly' adverbial suffix (Old English -lice). The adverb formation follows standard English word-building patterns for creating adverbs from adjectives.
Adding '-ly' to 'harebrained' transforms a folk slur against an animal into a way to describe how people act—showing how English lets us reuse prejudices as behavioral descriptions, hiding the original unfairness beneath layers of grammatical abstraction.
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