late 15c., "rough, coarse," unexplained alteration of Middle English boistous (c. 1300) "rough, coarse, clumsy, violent," which is 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.
Of persons, "turbulent, clamorous," 1560s; "Orig. in a distinctly bad sense, but gradually passing into ... [']Abounding in rough but good-natured activity bordering upon excess[']" [OED] by 1680s. Related: Boisterously; boisterousness.