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

While past reports have highlighted the dangers of deforestation, this study spotlights a more insidious threat: fire-driven degradation that erodes forest integrity without necessarily clearing it.

Read more on Science Daily

The corruption revelations tend to range from the somewhat venal — party bigwigs living on limited government salaries enjoying lavish lifestyles — to more insidious allegations of Morena officials in league with organized crime.

Read more on Los Angeles Times

Adams warned that "insidious forces" were pushing "divisive agendas" in city politics.

Read more on BBC

The insidious part is that this flattening won’t be noticeable overnight.

Read more on Salon

Starring in writer-director Alex Russell’s “Lurker,” a gripping psychological thriller that explores the insidious parasocial bond between a rising pop star and a seemingly meek retail employee, which is out in theaters Friday.

Read more on Los Angeles Times

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.