adjective in·sid·i·ous \in-ˈsi-dē-əs\

Definition of insidious

  1. 1a :  awaiting a chance to entrap :  treacherousb :  harmful but enticing :  seductive insidious drugs

  2. 2a :  having a gradual and cumulative effect :  subtle the insidious pressures of modern lifeb of a disease :  developing so gradually as to be well established before becoming apparent





Examples of insidious in a sentence

  1. 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

  2. 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

  3. 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

  4. Most people with this insidious disease have no idea that they are infected.

Recent Examples of insidious from the web

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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.”

Origin and Etymology of insidious

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

First Known Use: 1545

INSIDIOUS Defined for English Language Learners


adjective in·sid·i·ous \in-ˈsi-dē-əs\

Definition of insidious for English Language Learners

  • : causing harm in a way that is gradual or not easily noticed

Medical Dictionary


adjective in·sid·i·ous \in-ˈsid-ē-əs\

Medical Definition of insidious

  1. :  developing so gradually as to be well established before becoming apparent an insidious disease



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to criticize severely

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