in·​sid·​i·​ous | \in-ˈsi-dē-əs \

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

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 Less glam rock and more Agatha Christie, the simple story of one day in the Melrose family's life unfolds in the creaky mansion like a horror film, as insidious threats lurk at every turn. Kelly Lawler, USA TODAY, "Review: In 'Patrick Melrose,' Benedict Cumberbatch is raw, messy and better than ever," 9 May 2018 Season 2 delves into the more insidious parts of racial politics, like alt-right Twitter and the ingrained whiteness of university secret societies. Rebecca Farley,, "Dear White People Season 2 Binge Club: Episodes 1 - 3 Recaps," 4 May 2018 More insidious than crash injuries, though, were new chronic complaints attributed to cycling. Aaron Gilbreath, Longreads, "The Wheel, the Woman, and the Human Body," 6 July 2018 Cassius’ thrilling escape from drudgery and poverty reveals itself as simply a more insidious form of entrapment, one that sends poisonous tendrils into every corner of his life. Justin Chang,, "Review: Boots Riley's 'Sorry to Bother You' is an arrestingly surreal satire on class rage and cultural identity," 5 July 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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Last Updated

10 Dec 2018

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Time Traveler for insidious

The first known use of insidious was in 1545

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English Language Learners Definition of insidious

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


in·​sid·​i·​ous | \in-ˈsid-ē-əs \

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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More from Merriam-Webster on insidious

Rhyming Dictionary: Words that rhyme with insidious

Spanish Central: Translation of insidious

Nglish: Translation of insidious for Spanish Speakers

Britannica English: Translation of insidious for Arabic Speakers

Comments on insidious

What made you want to look up insidious? Please tell us where you read or heard it (including the quote, if possible).


living or existing for a long time

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