insidious

American  

[in-sid-ee-uhs] / ɪnˈsɪd i əs /

adjective

1. intended to entrap or beguile.

   an insidious plan.

2. stealthily treacherous or deceitful.

   an insidious enemy.

   Synonyms:

   crafty, subtle, wily, cunning, artful

3. operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect.

   an insidious disease.

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insidious British  

/ ɪnˈsɪdɪəs /

adjective

1. stealthy, subtle, cunning, or treacherous

2. working in a subtle or apparently innocuous way, but nevertheless deadly

   an insidious illness

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Usage

What does insidious mean? Insidious describes something as being sneaky or being secretly dangerous or harmful. Insidious also describes something as being corrupting or intending to entrap. The word insidious is used to describe something as being stealthy, treacherous, or operating in the shadows, as in He came up with an insidious plan to rob the jewelry store. Insidious is always associated with something negative, implying something is bad or is going to cause harm. Insidious is also often used to describe something as appearing harmless or nonthreatening but subtly causing serious damage, as in The insidious language in the book radicalized many readers without them even knowing it. Related to this sense, insidious is used in medicine to refer to diseases that slowly get worse and worse without any noticeable signs that they are there at all. Cancer is often described as insidious because it is often found when it is too late to treat it. Example: The investigators were after an insidious group of hackers who were stealing government secrets.

Other Word Forms

• insidiously adverb
• insidiousness noun
• uninsidious adjective
• uninsidiously adverb
• uninsidiousness noun

Etymology

Origin of insidious

First recorded in 1535–45; from Latin insidiōsus “deceitful,” from insidi(ae) (plural) “an ambush” (from insidēre “to sit in”) + -ōsus -ous; sit ¹

Example Sentences

An unseen chorus acts like another orchestral element, echoing the singers and amplifying the music’s insidious power.

From The Wall Street Journal • Apr. 8, 2026

It’s a subtle shift where “treat yourself” moments — like frequent dinners out or luxury upgrades — slowly morph into consistent, insidious monthly expenses.

From MarketWatch • Apr. 7, 2026

Times columnist Patt Morrison, Smoglandia traces the rise, impact, and retreat of the city’s most insidious pollution.

From Los Angeles Times • Mar. 26, 2026

"The cold is a very insidious enemy, it should not be underestimated," she said.

From Barron's • Feb. 16, 2026

They were designed for winter wear, when treacherous drafts came down chimneys and insidious currents of deadly cold found their way through keyholes.

From "The Awakening" by Kate Chopin

Related Words

• dangerous
• subtle