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.

---

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

---

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

Derived Forms

• uninsidiousness
  noun
• uninsidiously
  adverb
• uninsidious
  adjective
• insidiously
  adverb
• insidiousness
  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; see also sit ¹

Explanation

If something is slowly and secretly causing harm, it's insidious — like the rumors no one seems to listen to until suddenly someone's reputation is ruined. Insidious is related to the Latin noun, īnsidiae meaning "ambush" which comes from the Latin verb, īnsidēre "to lie in wait for." This is very fitting as an insidious rumor or problem is one whose negative effect is not realized until the damage is already done. Similarly, an insidious disease develops internally without symptoms, so that you don't realize right away that you are sick.

Example Sentences

One might assume that if these bias response teams were so insidious and widespread, there would be other cases in which students had been unfairly targeted by public universities, providing a clearer instance for standing.

From Slate • Jun. 2, 2026

That’s why the Rededicate 250 event is so insidious — even if the speakers behave themselves and don’t say anything too controversial on stage.

From Salon • May 15, 2026

"They are insidious; they will do what they want, and it will not save us, in any case," added the 71-year-old, who was displaced by fighting in eastern Ukraine.

From Barron's • Apr. 30, 2026

The intrusive thoughts of OCD, often called Pure-O or Pure OCD, are sometimes so insidious and about taboo subjects that sufferers do not admit to experiencing them.

From BBC • Apr. 29, 2026

The abrupt stillness seemed alien and artificial, a little insidious.

From "Catch-22" by Joseph Heller

Related Words

• dangerous
• subtle