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 While the comparisons seem somewhat obvious, The Assistant is about much more than the insidious effect of one man. Sarah Midkiff, refinery29.com, "The Assistant‘s Story Is Actually Far Bigger Than Harvey Weinstein," 30 Jan. 2020 Glyphosate ― the key ingredient in Roundup ― is an insidious herbicide. Howard Garrett, Dallas News, "How to remove a tree stump the organic way," 30 Jan. 2020 Despite the romantic image of coups as sudden insurgencies, usually military in nature, the reality is usually more prosaic and insidious. Garry Kasparov, The New York Review of Books, "A Popular Front to Stop Trump," 28 Jan. 2020 And there’s something amusing about how this Astros skulduggery can be traced to the growth of instant replay, an insidious development that is ruining game flow in every single sport. Jason Gay, WSJ, "Bang! Bang! You’re Suspended! Trashy Scandal Has Baseball Over a Barrel," 15 Jan. 2020 The insidious germ is known for creeping around healthcare facilities and infecting vulnerable patients, causing invasive infections marked by nondescript fever and chills. Beth Mole, Ars Technica, "Deadly fungus became resistant to all existing drugs in 3 unlinked US patients," 9 Jan. 2020 The Bureau is located within the shifting, ever-evolving Oldest House, which hosts an insidious organism known as the Hiss, which may end up holding the key to the disappearance of Jesse's brother as well as a host of other anomalies in the world. Brittany Vincent, CNN Underscored, "These are our favorite video games of 2019," 26 Dec. 2019 Hospitals, school districts, government agencies and businesses are increasingly falling victim to ransomware, which the Federal Bureau of Investigation describes as an insidious type of malware that encrypts or locks up valuable digital files. Laura Garcia, ExpressNews.com, "Cyber attack shuts down computers at San Antonio mental health provider," 24 Dec. 2019 Over the last 25 years or so, the simple interpretation of the law has been eroded by complicating court decisions and insidious little exceptions written into the law by our Legislature. al, "All I want for Christmas is a public records law that works," 21 Dec. 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

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

Statistics for insidious

Last Updated

19 Feb 2020

Cite this Entry

“Insidious.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/insidious. Accessed 22 Feb. 2020.

Keep scrolling for more

More Definitions for insidious

insidious

adjective
How to pronounce insidious (audio)

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

See Definitions and Examples »

Get Word of the Day daily email!

Test Your Vocabulary

More Confusing Words—Quiz

  • cats on impossible timber
  • The magician ______ moved the selected card to the top of the deck.
Spell It

Can you spell these 10 commonly misspelled words?

TAKE THE QUIZ
Add Diction

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!