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):

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

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "mischievousness":
   annoying, belie, color, devilment, devilry, deviltry, diablerie,
   distort, evil, falsify, garble, harm, hurt, impishness, injury,
   mischief, miscolor, misstate, odiousness, offensiveness, pervert,
   roguery, roguishness, sportiveness, teasing, twist, waggery,
   waggishness, warp