The Collaborative International Dictionary of English v.0.48:

Malicious \Ma*li"cious\, a. [Of. malicius, F. malicieux, fr. L.
   malitiosus. See Malice.]
   1. Indulging or exercising malice; harboring ill will or
      enmity.
      [1913 Webster]

            I grant him bloody, . . .
            Sudden, malicious, smacking of every sin
            That has a name.                      --Shak.
      [1913 Webster]

   2. Proceeding from hatred or ill will; dictated by malice;
      as, a malicious report; malicious mischief.
      [1913 Webster]

   3. (Law) With wicked or mischievous intentions or motives;
      wrongful and done intentionally without just cause or
      excuse; as, a malicious act.
      [1913 Webster]

   Malicious abandonment, the desertion of a wife or husband
      without just cause. --Burrill.

   Malicious prosecution or Malicious arrest (Law), a wanton
      prosecution or arrest, by regular process in a civil or
      criminal proceeding, without probable cause. --Bouvier.
      [1913 Webster]

   Syn: Ill-disposed; evil-minded; mischievous; envious;
        malevolent; invidious; spiteful; bitter; malignant;
        rancorous; malign.
        [1913 Webster] -- Ma*li"cious*ly, adv. --
        Ma*li"cious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

malicious
    adj 1: having the nature of or resulting from malice; "malicious
           gossip"; "took malicious pleasure in...watching me
           wince"- Rudyard Kipling [ant: unmalicious]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "malicious":
   acrid, antagonistic, antipathetic, baleful, baneful, belligerent,
   bitchy, bitter, catty, caustic, clashing, colliding, conflicting,
   cussed, deleterious, despiteful, detrimental, envious, evil,
   full of hate, green, green-eyed, harmful, hateful, hostile,
   iniquitous, invidious, jealous, malefic, maleficent, malevolent,
   malign, malignant, mean, nasty, noxious, ornery, pernicious, petty,
   poison-pen, poisonous, quarrelsome, rancorous, repugnant,
   set against, sore, spiteful, venomous, vicious, virulent,
   vitriolic, wicked



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MALICIOUS. With bad, and unlawful motives; wicked.