per·​ni·​cious pər-ˈni-shəs How to pronounce pernicious (audio)
: highly injurious or destructive : deadly
archaic : wicked
perniciously adverb
perniciousness noun

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insidious, sinister, or pernicious?

Few would choose to be associated with people or things that are insidious, sinister, or pernicious; all three of these words have decidedly unpleasant meanings, each with its own particular shade of nastiness.

Insidious comes from a Latin word for “ambush” (insidiae), which is fitting, as this word often carries the meanings “deceitful,” “stealthy,” or “harmful in an imperceptible fashion.” The first two meanings may be applied to people or things (“an insidious enemy,” “an insidious plot”), while the last is usually applied to things (“insidious problems,” “insidious sexism”), in particular to the gradual progress of a disease (“an insidious malignancy”).

Sinister comes from a Latin word meaning “on the left side, unlucky, inauspicious.” Although it is commonly used today in the sense “evil” (“a sinister cult leader”; “a sinister plot”), it may also suggest an ominous foreshadowing of some unfavorable turn of events (“a sinister omen”).

Pernicious has largely stayed true to its etymological root, the Latin noun pernicies “ruin, destruction.” Its original meaning in English, “highly injurious or destructive,” usually applies to things (“pernicious apathy,” “pernicious effects”) and medical conditions (“pernicious fever,” “pernicious anemia”). When applied to people, pernicious means “wicked.”

Choose the Right Synonym for pernicious

pernicious, baneful, noxious, deleterious, detrimental mean exceedingly harmful.

pernicious implies irreparable harm done through evil or insidious corrupting or undermining.

the claim that pornography has a pernicious effect on society

baneful implies injury through poisoning or destroying.

the baneful notion that discipline destroys creativity

noxious applies to what is both offensive and injurious to the health of a body or mind.

noxious chemical fumes

deleterious applies to what has an often unsuspected harmful effect.

a diet found to have deleterious effects

detrimental implies obvious harmfulness to something specified.

the detrimental effects of excessive drinking

Examples of pernicious in a Sentence

The notion that poll data are a legitimate form of news has to be one of the most pernicious tenets of late-twentieth-century American journalism … Barbara Ehrenreich, Nation, 20 Nov. 1995
The more it [the Papacy] took part in the temporal conflicts with consistently pernicious result, the more impotent among the monarchs it revealed itself … Barbara W. Tuchman, The March of Folly, 1984
At its most pernicious, paper entrepreneurialism involves little more than imposing losses on others for the sake of short-term profits for the firm. Robert B. Reich, Atlantic, March 1983
More pernicious still has been the acceptance of the author's controversial ideas by the general public. the pernicious effects of jealousy She thinks television has a pernicious influence on our children.
Recent Examples on the Web Academic opportunists the past week showed once more how pernicious, naïve misinformation can catch fire and consume the truth, especially when dressed with the veneer of academic credibility. Jeffrey Sonnenfeld, Fortune, 2 Feb. 2023 Most of us can agree the world is in a perilous state, with natural disasters multiplying, pernicious new viruses continually emerging, the planet steadily overheating, and wars raging in constant rotation. David Rooney, The Hollywood Reporter, 1 Feb. 2023 See all Example Sentences for pernicious 

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'pernicious.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


Middle English, from Anglo-French, from Latin perniciosus, from pernicies destruction, from per- + nec-, nex violent death — more at noxious

First Known Use

15th century, in the meaning defined at sense 1

Time Traveler
The first known use of pernicious was in the 15th century

Dictionary Entries Near pernicious

Cite this Entry

“Pernicious.” Dictionary, Merriam-Webster, Accessed 28 May. 2024.

Kids Definition


per·​ni·​cious pər-ˈnish-əs How to pronounce pernicious (audio)
: very destructive or harmful
a pernicious disease
perniciously adverb

Medical Definition


per·​ni·​cious pər-ˈnish-əs How to pronounce pernicious (audio)
: highly injurious or destructive : tending to a fatal issue : deadly
pernicious disease

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