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

Keep scrolling for more

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.
See More

Recent Examples on the Web

But recent disinformation campaigns—especially ones that originate with coordinated agencies in Russia or China—have been far more sweeping and insidious. The Editors, Scientific American, "Everyone Is an Agent in the New Information Warfare," 26 Aug. 2019 Hurricanes are sudden and violent; sea-level rise is insidious and creeping. David Campany, The New Yorker, "Life in Miami on the Knife’s Edge of Climate Change," 18 Aug. 2019 However, conservation experts are even more concerned with habitat loss and other less violent, but thoroughly insidious dangers to the species. Doug Johnson, Quartz, "Elephants and whales could give us the cure for cancer—unless we keep killing them," 14 Aug. 2019 The content of their conversation, combined with internal memos released and explicated by an anonymous internal whistleblower, were meant to demonstrate a pervasive and insidious left-wing bias at Google. John Hirschauer, National Review, "Google Crusades for ‘Fairness’," 3 July 2019 Some elements of the Left, such as Antifa, have openly embraced violence as a means of suppressing unpopular political speech, but Senator Booker’s more polite model is more common and, if anything, more insidious. Kevin D. Williamson, National Review, "The Reflex toward Illiberal Democracy," 7 Aug. 2019 Today the reality is less cinematic but more insidious. Peter Keough, BostonGlobe.com, "In vogue, online, at Woodstock," 25 July 2019 Organs can be rejected by the immune system immediately, which is called acute rejection; the more insidious problem is long-term rejection, in which the body slowly begins to kick the organ out. Caitlin Dwyer, Longreads, "Shared Breath," 25 July 2019 Then there are the more insidious encroachments on our rights that happen quietly, attracting little attention and sunlight. Esther J. Cepeda, The Mercury News, "Cepeda: New ICE plan threatens immigrants and communities," 11 July 2019

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.

See More

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

Keep scrolling for more

Keep scrolling for more

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

Keep scrolling for more

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).

WORD OF THE DAY

suitable to be imparted to the public

Get Word of the Day daily email!

Test Your Vocabulary

Where in the World? A Quiz

  • peter bruegel tower of babel painting
  • What language does pajama come from?
True or False

Test your knowledge - and maybe learn something along the way.

TAKE THE QUIZ
Citation

Test Your Knowledge - and learn some interesting things along the way.

TAKE THE QUIZ
Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!