Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Though I perchance am vicious in my guess. --Shak.

            The title of these lords was vicious in its origin.
                                                  --Burke.

            A charge against Bentley of vicious reasoning. --De
                                                  Quincey.

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

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

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

   4. Not correct or pure; corrupt; as, vicious language;
      vicious idioms.

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

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

   Syn: Corrupt; faulty; wicked; depraved. -- {Vi"cious*ly},
        adv. -- {Vi"cious*ness}, n.

Source : WordNet®

viciousness
     n : the trait of extreme cruelty [syn: {ferociousness}, {brutality},
          {savageness}, {savagery}]