insidious

adjective

in·​sid·​i·​ous in-ˈsi-dē-əs How to pronounce insidious (audio)
1
a
: 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
2
a
: awaiting a chance to entrap : treacherous
b
: harmful but enticing : seductive
insidious drugs
insidiously adverb
insidiousness noun

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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.
Recent Examples on the Web My grandmother’s battle with HIV starkly unveiled to a 14-year-old me the insidious nature of stigma. Larada Lee, Essence, 9 Mar. 2024 But technological advancement and social media have created even more insidious conditions than the tabloid industry as everyone now has the tools to spread private information, and even fewer ethical considerations to make. Mankaprr Conteh, Rolling Stone, 24 Feb. 2024 Experts said the new regulations will make California a national leader in battling the insidious and deadly effects of lead in the workplace. Emily Alpert Reyes, Los Angeles Times, 16 Feb. 2024 Indeed, social media can be considered a more insidious form of visual clutter for those who prefer a cleaner, simpler work environment. Joshua Rapp Learn, Discover Magazine, 14 Feb. 2024 While traditional media operates within the bounds of human oversight and journalistic ethics, social media—which was once a space for discourse and public opinion—is increasingly influenced by the insidious presence of bots. Guy Tytunovich, Forbes, 28 Feb. 2024 The new research highlights the insidious toll police violence takes on the Black community. Claretta Bellamy, NBC News, 6 Feb. 2024 After realizing that Constance (Elizabeth Perkins) was an insidious influence, Joyce went rogue, joining Richie (Oscar Montoya), Shelly (Lennon Parham), Bambi (Jessica Lowe), and Tina (Idara Victor) to publish their own version of Minx. Maureen Lee Lenker, EW.com, 5 Jan. 2024 Although the film is more ambitious than the novel on some of the more insidious subtexts, Eli (Lina Leandersson) presents as a 12-year-old girl who happens to be a vampire looked after by an elderly man who procures that good stuff for her. Nicholas Bell, SPIN, 14 Feb. 2024

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'insidious.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

Etymology

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

First Known Use

1545, in the meaning defined at sense 2a

Time Traveler
The first known use of insidious was in 1545

Dictionary Entries Near insidious

Cite this Entry

“Insidious.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/insidious. Accessed 19 Mar. 2024.

Kids Definition

insidious

adjective
in·​sid·​i·​ous in-ˈsid-ē-əs How to pronounce insidious (audio)
1
a
: awaiting a chance to trap : treacherous
an insidious foe
b
: harmful but attractive
an insidious habit
2
: having an effect that develops gradually
an insidious disease
insidiously adverb
insidiousness noun

Medical Definition

insidious

adjective
in·​sid·​i·​ous in-ˈsid-ē-əs How to pronounce insidious (audio)
: developing so gradually as to be well established before becoming apparent
an insidious disease
insidiously adverb
Last Updated: - Updated example sentences
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