The Collaborative International Dictionary of English v.0.48:

Vicious \Vi"cious\, a. [OF. vicious, F. vicieux, fr. L.
   vitiosus, fr. vitium vice. See Vice a fault.]
   1. Characterized by vice or defects; defective; faulty;
      imperfect.
      [1913 Webster]

            Though I perchance am vicious in my guess. --Shak.
      [1913 Webster]

            The title of these lords was vicious in its origin.
                                                  --Burke.
      [1913 Webster]

            A charge against Bentley of vicious reasoning. --De
                                                  Quincey.
      [1913 Webster]

   2. Addicted to vice; corrupt in principles or conduct;
      depraved; wicked; as, vicious children; vicious examples;
      vicious conduct.
      [1913 Webster]

            Who . . . heard this heavy curse,
            Servant of servants, on his vicious race. --Milton.
      [1913 Webster]

   3. Wanting purity; foul; bad; noxious; as, vicious air,
      water, etc. --Dryden.
      [1913 Webster]

   4. Not correct or pure; corrupt; as, vicious language;
      vicious idioms.
      [1913 Webster]

   5. Not well tamed or broken; given to bad tricks; unruly;
      refractory; as, a vicious horse.
      [1913 Webster]

   6. Bitter; spiteful; malignant. [Colloq.]
      [1913 Webster]

   Syn: Corrupt; faulty; wicked; depraved.
        [1913 Webster] -- Vi"cious*ly, adv. --
        Vi"cious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

viciousness
    n 1: the trait of extreme cruelty [syn: ferociousness,
         brutality, viciousness, savagery]