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]

The Collaborative International Dictionary of English v.0.48:

Pernicious \Per*ni"cious\, a. [L. pernix, -icis.]
   Quick; swift (to burn). [R.] --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pernicious
    adj 1: exceedingly harmful [syn: baneful, deadly,
           pernicious, pestilent]
    2: working or spreading in a hidden and usually injurious way;
       "glaucoma is an insidious disease"; "a subtle poison" [syn:
       insidious, pernicious, subtle]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "pernicious":
   baleful, baneful, brutal, corroding, corrosive, corrupting,
   corruptive, counterproductive, damaging, deadly, death-bringing,
   deathful, deathly, deleterious, destructive, detrimental,
   devastating, disadvantageous, disserviceable, distressing, fatal,
   feral, harmful, hurtful, injurious, internecine, killing, lethal,
   malefic, maleficent, malevolent, malign, malignant, miasmatic,
   miasmic, mischievous, mortal, noisome, noxious, ominous,
   pestiferous, pestilent, pestilential, poisonous, prejudicial,
   ruinous, savage, scatheful, sinister, swart, toxic, venenate,
   veneniferous, venenous, venomous, vicious, virulent