Dictionary.com
definitions

pernicious

[per-nish-uhs]
adjective
  1. causing insidious harm or ruin; ruinous; injurious; hurtful: pernicious teachings; a pernicious lie.
  2. deadly; fatal: a pernicious disease.
  3. Obsolete. evil; wicked.

Origin of pernicious

1515–25; < Latin perniciōsus ruinous, equivalent to pernici(ēs) ruin (per- per- + -nici-, combining form of nex death, murder (stem nec-) + -iēs noun suffix) + -ōsus -ous
Related formsper·ni·cious·ly, adverbper·ni·cious·ness, nounun·per·ni·cious, adjectiveun·per·ni·cious·ly, adverb

Synonyms for pernicious

1. harmful, detrimental, deleterious, destructive, damaging, baneful, noxious, malicious. 2. lethal.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for perniciously

Contemporary Examples of perniciously

  • Not long ago, polite society had only begun to tolerate slamming the Electoral College as a perniciously quaint old institution.

    The Daily Beast logo
    Disunited and Without States

    James Poulos

    January 4, 2014

Historical Examples of perniciously

  • And probably I've behaved more foolishly and perniciously than ever you will.

  • In these matters his aims were generous, if his methods were perniciously mistaken.

    Shelley

    Francis Thompson

  • He explained that Allen had been a feeble speculator, but plausible, of personal good faith, and perniciously sanguine.

  • These indefinite expressions operated powerfully and perniciously on his imagination.

  • But Supermanism is heresy, and it rapidly spread over the world, most perniciously influencing all intellectual life.

British Dictionary definitions for perniciously

pernicious

adjective
  1. wicked or maliciouspernicious lies
  2. causing grave harm; deadly
Derived Formsperniciously, adverbperniciousness, noun

Word Origin for pernicious

C16: from Latin perniciōsus, from perniciēs ruin, from per- (intensive) + nex death
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for perniciously

pernicious

adj.

early 15c., from Middle French pernicios (13c., Modern French pernicieux) and directly from Latin perniciosus "destructive," from pernicies "destruction, death, ruin," from per- "completely" (see per) + necis "violent death, murder," related to necare "to kill," nocere "to hurt, injure, harm," noxa "harm, injury" (see noxious). Related: Perniciously; perniciousness.

Online Etymology Dictionary, © 2010 Douglas Harper
perniciously in Medicine

pernicious

[pər-nĭsh′əs]
adj.
  1. Tending to cause death or serious injury; deadly.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.