Source : Webster's Revised Unabridged Dictionary (1913)

Mischievous \Mis"chie*vous\, a.
   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. --
        {Mis"chie*vous*ness}, n.

Source : WordNet®

mischievousness
     n 1: an attribute of mischievous children [syn: {naughtiness}, {badness}]
     2: the trait of behaving like an imp [syn: {impishness}, {puckishness},
         {whimsicality}]
     3: reckless or malicious behavior that causes discomfort or
        annoyance in others [syn: {mischief}, {mischief-making}, {deviltry},
         {devilry}, {devilment}, {rascality}, {roguery}, {roguishness},
         {shenanigan}]