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late 15c., unexplained alteration of Middle English boistous (c.1300) "rough, coarse (as of food), clumsy, violent," of unknown origin, perhaps from Anglo-French bustous "rough (road)," which is perhaps from Old French boisteos "curved, lame; uneven, rough" (Modern French boiteux), itself of obscure origin. Another guess traces it via Celtic to Latin bestia. Used of persons from 1560s. Related: Boisterously; boisterousness.