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

So, this is a kind of insidious influence on our academic world. Fox News, "Has China been duping the US for nearly half a century?," 13 Aug. 2018 Perhaps privilege is a powerful drug (as insidious as sugar). Angela Helm, The Root, "Chicago PD Settle for $2.5 Million After Police Point Gun at 3-Year-Old," 4 July 2018 By 2012, Riley, then 41, started shopping a screenplay for a radical comedy about unions, displacement and the insidious effects of capitalism. Joseph Bien-kahn, WIRED, "Radical As Ever, Boots Riley Takes On the Tech Boom," 4 July 2018 But aid experts and U.N. officials say a more insidious form of warfare is also being waged in Yemen, an economic war that is exacting a far greater toll on civilians and now risks tipping the country into a famine of catastrophic proportions. Declan Walsh, The Seattle Times, "The tragedy of Saudi Arabia’s war in Yemen," 26 Oct. 2018 Today, colorism is still particularly insidious in film, fashion, and beauty—industries predicated on appearances. Yaminah Mayo, Glamour, "Khloé Kardashian Shouldn't Have to 'Block Out' Racist Remarks About True," 20 Sep. 2018 In the advisory, Radware researcher Pascal Geenens wrote: The attack is insidious in the sense that a user is completely unaware of the change. Dan Goodin, Ars Technica, "In-the-wild router exploit sends unwitting users to fake banking site," 10 Aug. 2018 For example, the sneaky Genio adware, DevilRobber currency mining malware, and the insidious Fruitfly malware that stole millions of images from infected Macs over a 13-year period all used synthetic clicks to bypass defense-in-depth warnings. Dan Goodin, Ars Technica, "Malware has no trouble hiding and bypassing macOS user warnings," 14 Aug. 2018 But something else is happening as well, something insidious and dangerous. Abby Gardner, Glamour, "Death Threats and Discrediting: The Treatment of Christine Blasey Ford Is a Reminder of What's at Stake for Sexual Assault Survivors," 21 Sep. 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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