Source : Webster's Revised Unabridged Dictionary (1913)

Pestilent \Pes"ti*lent\, a. [L. pestilens, -entis, fr. pestis
   pest: cf. F. pestilent.]
   Pestilential; noxious; pernicious; mischievous. ``Corrupt and
   pestilent.'' --Milton. ``What a pestilent knave is this
   same!'' --Shak.

Source : WordNet®

pestilent
     adj 1: exceedingly harmful [syn: {baneful}, {deadly}, {pernicious}]
     2: likely to spread and cause an epidemic disease; "a
        pestilential malignancy in the air"- Jonathan Swift;
        "plaguey fevers" [syn: {pestilential}, {pestiferous}, {plaguey}]