Source : Webster's Revised Unabridged Dictionary (1913)

Pernicious \Per*ni"cious\, a. [L. perniciosus, from pernicies
   destruction, from pernecare to kill or slay outright; per +
   necare to kill, slay: cf. F. pernicieux. Cf. {Nuisance},
   {Necromancy}.]
   Having the quality of injuring or killing; destructive; very
   mischievous; baleful; malicious; wicked.

         Let this pernicious hour Stand aye accursed in the
         calendar.                                --Shak.

         Pernicious to his health.                --Prescott.

   Syn: Destructive; ruinous; deadly; noxious; injurious;
        baneful; deleterious; hurtful; mischievous. --
        {Per*ni"cious*ly}, adv., -- {Per*ni"cious*ness}, n.

Source : WordNet®

perniciousness
     n : grave harmfulness or deadliness [syn: {toxicity}]