Etymology
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pestilent (adj.)

late 14c., "contaminated with dangerous disease; deadly, poisonous," from Latin pestilentem (nominative pestilens) "infected, unhealthy," from pestilis "of the nature of a plague," from pestis "deadly contagious disease" (see pest (n.)). Transferred sense of "mischievous, pernicious, hurtful to health or morals" is from 1510s; weakened sense of "troublesome" is from 1590s. Related: Pestilently.

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Definitions of pestilent

pestilent (adj.)
exceedingly harmful;
Synonyms: baneful / deadly / pernicious
pestilent (adj.)
likely to spread and cause an epidemic disease; "a pestilential malignancy in the air"- Jonathan Swift;
From wordnet.princeton.edu