Causing mischief; harmful; hurtful; -- now often applied where the evil is done carelessly or in sport; as, a mischievous child. "Most mischievous foul sin." Shak. This false, wily, doubling disposition is intolerably mischievous to society. South. Syn. -- Harmful; hurtful; detrimental; noxious; pernicious; destructive. -- Mis"chie*vous*ly, adv. ...