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

These innocent, innocuous, not-at-all-strange things become crucial plot points as the dark force known as the Mind Flayer returns in an insidious new form. Tom Gliatto, PEOPLE.com, "Stranger Things 3 Is Even Better Than Season 2: Read PEOPLE's Review of Netflix's Fun-Creepy Hit," 1 July 2019 This sick longing for security is a dangerous thing, Meg, as insidious as the radiation from a nuclear explosion. Meghan Cox Gurdon, WSJ, "Children’s Books: Stirring Up Fresh Life Endlessly," 21 Dec. 2018 Industry gurus talk about the insidious biological effects of emotional stress while shilling for stress-inducing lifestyles of strict diet, exercise, and meditation routines. Brennan Kilbane, GQ, "The Least Stressful Way to Get Rid of Stress," 16 May 2018 The impact this will have on the bedrock principles of democratic U.S. governance is much more insidious. Adam Weinstein, The New Republic, "Who Is Actually Running the U.S. Military’s Iran Efforts?," 18 June 2019 The last, and perhaps most insidious, factor: the advent of social media and its effect on weddings. Sangeeta Singh-kurtz, Quartz, "Young Americans are racking up debt for Instagrammable weddings," 20 June 2019 Purdue knew its opioids were addictive and its sales tactics were insidious. WSJ, "States Are Right to Pursue Big Opioid Maker," 19 June 2019 But what’s most insidious about Draper’s proposal is the disproportionate distribution of wealth among the three new states. Michael Hiltzik, latimes.com, "Venture investor Tim Draper is back with a pointless plan to split California three ways," 22 June 2018 But these are insidious threats being propagated on these Web platforms. Washington Post, "The Health 202: Trump's tariffs could defeat his goal of lowering drug prices," 5 Apr. 2018

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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More Definitions for insidious

insidious

adjective

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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