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; 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

Last weekend it took nearly $20m according to IMDB, compared to $27m for the new Indiana Jones instalment and $33m for the top-grossing Insidious: The Red Door.

From BBC • Jul. 13, 2023

In its second weekend in theaters, the Disney release was usurped by another franchise fifth – “ Insidious: The Red Door.”

From Seattle Times • Jul. 9, 2023

I have to ask you one more thing, because I'm a big "Insidious" fan.

From Salon • Jun. 23, 2023

Until then, the hard-working Australian screenwriter had been known as the co-creator, with director James Wan, of the wildly popular Saw and Insidious franchises.

From Washington Post • May 30, 2018

Insidious questions were propounded to Mrs. Stannard, Mrs. Truscott, and Miss Sanford, only to mystify him the more.

From Marion's Faith. by King, Charles

Related Words

• dangerous
• subtle