late 15c., unexplained alteration of M.E. boistous (c.1300) "rough, coarse (as of food), clumsy, violent," of unknown origin, perhaps from Anglo-Fr. bustous "rough (road)," which is perhaps from O.Fr. boisteos "curved, lame; uneven, rough" (Mod.Fr. boiteux), itself of obscure origin. Another guess traces it via Celtic to L. bestia. Used of persons from 1560s.