insidious

[in-sid-ee-uhs]

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

/ ɪnˈsɪdɪəs /

adjective

1. stealthy, subtle, cunning, or treacherous

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

   an insidious illness

Other Word Forms

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

Word History and Origins

Origin of insidious¹

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

Word History and Origins

Origin of insidious¹

C16: from Latin insidiōsus cunning, from insidiae an ambush, from insidēre to sit in; see _insessorial

Example Sentences

More insidious groupthink has taken hold inside the NFL: only quarterbacks can win the league’s Most Valuable Player award.

Read more on The Wall Street Journal

Hatred of minorities: Fascism lacks a rationale and focus for its followers’ emotional intensity without an insidious internal enemy.

Read more on Salon

But an equally insidious carbon tax has been quietly advancing in county courthouses across the U.S. in the form of climate lawfare targeted almost exclusively at American industry.

Read more on The Wall Street Journal

On Saturday, Scotland Yard said group-based offending was "particularly insidious and devastating in its profound impact on the children affected".

Read more on BBC

One of the most "insidious" problems, Natalie said, was as the pain increased, ketamine became one of the only ways of numbing it - causing further damage.

Read more on BBC

Related Words

• dangerous 
• subtle 

When To Use

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.