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 George’s best friend and eventual love interest, Izzie Stevens (Katherine Heigl), adds an even more insidious theme to the in-series discourse around Callie. Ariana Romero, refinery29.com, "Callie Torres Deserved Better From Grey’s Anatomy," 1 Feb. 2021 Joshua Jackson is set to play the insidious surgeon himself, Dr. Christopher Duntsch. Megan Ditrolio, Marie Claire, "The Best True Crime Documentaries of 2021," 6 Jan. 2021 As of Wednesday afternoon, 25,033 Californians had lost their lives to the insidious virus since the start of the pandemic. Chronicle Staff, SFChronicle.com, "Coronavirus updates from the Bay Area: Dec. 24-30," 1 Jan. 2021 As the nation’s health is besieged by the coronavirus, another invisible yet more insidious infectious disease threatens the very fiber of our democracy. Arkansas Online, "OPINION | ART ENGLISH: Seeds of white nationalism," 11 Oct. 2020 Rarely, if ever before in history have so many high-value victims been compromised by such a singular, insidious hacking technique. Wired Staff, Wired, "The Most Dangerous People on the Internet in 2020," 30 Dec. 2020 When the coronavirus began its dramatic and insidious spread, with New York City at the epicenter nationally, Hernandez became sick. Liz Willen, USA TODAY, "‘The heart of a nurse’: Mother of 4 overcomes COVID-19, other obstacles to earn degree," 25 Dec. 2020 The interplay between cortisol and glucose is complex and insidious, triggering metabolic changes that can lead to diabetes and other chronic diseases. Akilah Johnson, ProPublica, "How COVID-19 Hollowed Out a Generation of Young Black Men," 22 Dec. 2020 Taking into account some models’ ability to filter out more harmful and more insidious particles like viruses and VOCs, an air purifier can be one of the easiest and most subtle ways to improve your daily quality of life. Popular Science, "Best air purifier: Fight allergens, smoke, and germs for cleaner indoor air," 16 Dec. 2020

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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Last Updated

23 Feb 2021

Cite this Entry

“Insidious.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/insidious. Accessed 2 Mar. 2021.

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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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