late 15c., boistreous, "rough, coarse," an unexplained alteration of Middle English boistous (c. 1300) "rough, coarse, clumsy, violent," which itself is of unknown origin, perhaps from Anglo-French bustous "rough (road)," itself 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.
Of persons, "turbulent, clamorous," 1560s; OED says originally "in a distinctly bad sense," but by 1680s gradually passing into "abounding in rough but good-natured activity bordering upon excess." Related: Boisterously; boisterousness.