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

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.
Recent Examples on the Web Mining — particularly for gold, the mineral that draws the greatest interest in the Amazon — has an insidious effect on the environment. Washington Post, 19 Mar. 2022 Inflation’s Impact Rising inflation can have an insidious effect on the economy. Jj Kinahan, Forbes, 18 Jan. 2022 This second dose of venom has the insidious effect of changing the roach from a violently struggling (and dangerous) opponent into a compliant and pacified host—that is, a zombie. Kenneth C. Catania, Scientific American, 1 Feb. 2021 As Brussels focuses on aggression by Moscow, a more insidious threat to democracy has strengthened its hand in Budapest. Yasmeen Serhan, The Atlantic, 11 Apr. 2022 Diabetes is an insidious disease that significantly increases the risks of premature blindness, stroke, and circulatory and neurological problems that can lead to infections requiring amputation of gangrenous toes and feet. New York Times, 3 Apr. 2022 Both images are sellable, but the second is more insidious. Margaret Talbot, The New Yorker, 14 Mar. 2022 These days, Lowe says allegations typically follow a similar arc to Brougher's at Pinterest, where discrimination was more insidious, taking the form of exclusion from important meetings or a gender pay gap. Emma Hinchliffe, Fortune, 15 Feb. 2022 But more insidious than any police misconduct is the Trudeau government’s weaponization of emergency powers. Nate Hochman, National Review, 26 Feb. 2022 See More

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.

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

Learn More About insidious

Time Traveler for insidious

Time Traveler

The first known use of insidious was in 1545

See more words from the same year

Dictionary Entries Near insidious

insidiate

insidious

insight

See More Nearby Entries 

Statistics for insidious

Last Updated

18 May 2022

Cite this Entry

“Insidious.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/insidious. Accessed 26 May. 2022.

Style: MLA
MLACheck Mark Icon ChicagoCheck Mark Icon APACheck Mark Icon Merriam-WebsterCheck Mark Icon

More Definitions for insidious

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

WORD OF THE DAY

Test Your Vocabulary

Name That Food

  • a-light
  • Name these cookies!
How Strong Is Your Vocabulary?

Test your vocabulary with our 10-question quiz!

TAKE THE QUIZ
Universal Daily Crossword

A daily challenge for crossword fanatics.

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!