insidious

adjective in·sid·i·ous \ in-ˈsi-dē-əs \

Updated on: 17 Apr 2018

Definition of insidious

1 a : awaiting a chance to entrap : treacherous

b : harmful but enticing : seductive

insidious drugs

2 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

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insidiously

adverb

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insidiousness

noun

See insidious defined for English-language learners

See insidious defined for kids

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 of insidious from the Web

By pursuing public life, women invite assault is one of the most insidious myths out there.

— megan johnson, PEOPLE.com, "Gretchen Carlson Indicates Swimsuit Competition May Be Removed from Miss America Pageants," 5 Apr. 2018
The show’s writers — Dracula Untold and Gods of Egypt’s Matt Sazama and Burk Sharpless are behind this reboot — do seem somewhat aware of the insidious nature of this line of thinking.

— andrew liptak, The Verge, "Netflix’s Lost in Space undermines a strong premise with dumb characters," 3 Apr. 2018
Vapes can also be helpful for weaning yourself off a more insidious smoking habit, as most e-liquids come in different strengths, allowing users to titrate down.

— julia felsenthal, Vogue, "Health Concerns About Vaping are Growing—Here's Why," 2 Apr. 2018
Tyson's tax-cut push is insidious on another front.

— the kansas city star's editorial board, kansascity, "Why are Kansas lawmakers pushing for more tax cuts instead of funding schools? | The Kansas City Star," 4 Mar. 2018
Corrosion and rust are insidious foes, and soil shifts can break subterranean communication lines.

— joe pappalardo, Popular Mechanics, "Meet the Airmen Who Watch Over America's Nukes—And Await the Unthinkable," 23 Mar. 2011
Andrew Bosworth’s memo may seem insidious, but its logic used to be very popular.

— robinson meyer, The Atlantic, "Facebook's Ideological Imperialism," 30 Mar. 2018
Axios reported on Wednesday morning that, while Facebook may increasingly be in the sights of legislators and regulators, Trump is more concerned with Amazon’s insidious influence on American life.

— alex shephard, The New Republic, "This is why Trump is “obsessed” with Amazon.," 28 Mar. 2018
She’d been born with hepatitis C, an insidious liver disease her mother got shooting up drugs.

— laura ungar, The Courier-Journal, "A serious liver ailment is stalking Kentucky's children. But they aren't getting care," 22 Mar. 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.

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.”