The Collaborative International Dictionary of English v.0.48:

Mischievous \Mis"chie*vous\ (m[i^]s"ch[-e]*v[u^]s), 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.
   [1913 Webster]

         This false, wily, doubling disposition is intolerably
         mischievous to society.                  --South.
   [1913 Webster]

   Syn: Harmful; hurtful; detrimental; noxious; pernicious;
        destructive.
        [1913 Webster] -- Mis"chie*vous*ly, adv. --
        Mis"chie*vous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

mischievous
    adj 1: naughtily or annoyingly playful; "teasing and worrying
           with impish laughter"; "a wicked prank" [syn: arch,
           impish, implike, mischievous, pixilated,
           prankish, puckish, wicked]
    2: deliberately causing harm or damage; "mischievous rumors and
       falsehoods"

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "mischievous":
   annoying, arch, artful, bad, baleful, baneful, bothering,
   bothersome, corroding, corrosive, corrupting, corruptive,
   counterproductive, damaging, dangerous, deadly, deleterious,
   destructive, detrimental, devilish, disadvantageous,
   disserviceable, distressing, elfish, elvish, evil, foolish, foxy,
   frolicsome, full of mischief, harmful, hazardous, high-spirited,
   hurtful, ill, ill-behaved, impish, injurious, insidious, irking,
   irksome, knavish, larkish, lethal, malefic, malevolent, malicious,
   malign, malignant, misbehaving, mischief-loving, naughty, noisome,
   noxious, ominous, paw, perilous, pernicious, playful, poisonous,
   prankish, pranksome, pranky, precarious, prejudicial, puckish,
   rascally, risky, roguish, scampish, scapegrace, scatheful, sly,
   spiteful, sportive, toxic, trickish, tricksy, tricky, venenate,
   veneniferous, venenous, venomous, vexatious, vexing, vicious,
   virulent, waggish, wicked