insidious

adjective
in·​sid·​i·​ous | \ in-ˈsi-dē-əs How to pronounce insidious (audio) \

Definition of insidious

1a : having a gradual and cumulative effect : subtle the insidious pressures of modern life
b of a disease : developing so gradually as to be well established before becoming apparent
2a : awaiting a chance to entrap : treacherous
b : harmful but enticing : seductive insidious drugs

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Other Words from insidious

insidiously adverb
insidiousness noun

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.”

Examples of insidious in a Sentence

But the litigation is also prompting a subtle and insidious change in the way that medicine is practiced, which affects anyone who consults a health professional, even if they would not dream of setting foot in a lawyer's office. It is known as "defensive medicine." — Geoff Watts, New Scientist, 23–29 Oct. 2004 Spin is sometimes dismissed as a simple euphemism for lying. But it's actually something more insidious: indifference to the truth. — Michael Kinsley, Time, 25 Dec. 2000–1 Jan. 2001 As these boats aged and bedding compounds deteriorated, the water torture began, which led to rot, corrosion, and other insidious problems. — Ralph Naranjo, Cruising World, April 1999 Most people with this insidious disease have no idea that they are infected.
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Recent Examples on the Web By allowing anyone to post definitions (users can up or down vote their favorite ones) Peckham opened the door for the most insidious among us. Wired, "What Happened to Urban Dictionary?," 11 Sep. 2019 While the taking of property may appear to be the most obvious violation of the Eighth Commandment, Ms. Scott sees the more insidious withholding of seemingly small things to be a truer threat. Mary Beth Mccauley, The Christian Science Monitor, "‘Thou shalt not steal’: Even someone else’s joy, says one educator," 13 Dec. 2019 More insidious is moisture that opens the door to mildew. Popmech Editors, Popular Mechanics, "How To Winterize Your Boat for the Cold Months Ahead," 21 Nov. 2019 There are more insidious limitations, however, than that example implies. Kate Sheridan, STAT, "Social connections first, science second: How biotech’s recipe for success has its limits," 31 Oct. 2019 Much of the film is devoted to the frenetic battle between Lio and Galo, until a more insidious villain is unmasked. Charles Solomon, Los Angeles Times, "Review: The Day-Glo fantasia of Hiroyuki Imaishi’s ‘Promare’ shuns realism for bright action," 19 Sep. 2019 There’s also a more insidious, paradoxical cultural force at work. Collier Meyerson, WIRED, "The Last Black Man Searches for ‘Authenticity’ but There Is None," 6 June 2019 The reinforcement is all the more insidious for being intermittent. Kate Bachelder Odell, WSJ, "It’s Not Too Late to Quit Social Media," 25 Jan. 2019 That illusion has been pierced since 2016, but the myth that seems poised to replace it—that technology can in fact automate away bias and politics itself—is no less insidious. Ingrid Burrington, The New Republic, "How We Misremember the Internet’s Origins," 29 Oct. 2019

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'insidious.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of insidious

1545, in the meaning defined at sense 2a

History and Etymology for insidious

Latin insidiosus, from insidiae ambush, from insidēre to sit in, sit on, from in- + sedēre to sit — more at sit

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Time Traveler for insidious

Time Traveler

The first known use of insidious was in 1545

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Statistics for insidious

Last Updated

8 Jan 2020

Cite this Entry

“Insidious.” The Merriam-Webster.com Dictionary, Merriam-Webster Inc., https://www.merriam-webster.com/dictionary/insidious?pronunciation&lang=en_us&dir=i&file=insidi01. Accessed 20 January 2020.

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More Definitions for insidious

insidious

adjective
How to pronounce insidious (audio)

English Language Learners Definition of insidious

formal : causing harm in a way that is gradual or not easily noticed

insidious

adjective
in·​sid·​i·​ous | \ in-ˈsid-ē-əs How to pronounce insidious (audio) \

Medical Definition of insidious

: developing so gradually as to be well established before becoming apparent an insidious disease

Other Words from insidious

insidiously adverb

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Comments on insidious

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not to be intimidated or subdued

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