The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Let this pernicious hour
         Stand aye accursed in the calendar.      --Shak.
   [1913 Webster]

         Pernicious to his health.                --Prescott.
   [1913 Webster]

   Syn: Destructive; ruinous; deadly; noxious; injurious;
        baneful; deleterious; hurtful; mischievous.
        [1913 Webster] -- Per*ni"cious*ly, adv., --
        Per*ni"cious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

perniciousness
    n 1: grave harmfulness or deadliness [syn: perniciousness,
         toxicity]