[ in-sid-ee-uhs ]
/ ɪnˈsɪd i əs /
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See synonyms for: insidious / insidiously / insidiousness on Thesaurus.com


intended to entrap or beguile: an insidious plan.
stealthily treacherous or deceitful: an insidious enemy.
operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect: an insidious disease.

SYNONYMS FOR insidious

1 corrupting.



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Origin of insidious

1535–45; <Latin insidiōsus deceitful, equivalent to insidi(ae) (plural) an ambush (derivative of insidēre to sit in or on) + -ōsus-ous; see sit1



insidious , invidious
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021


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.

Where does insidious come from?

The first records of insidious come from around 1535. It comes from the Latin insidiōsus, meaning “deceitful.”

The word insidious is often used in the media and popular culture to describe conspiracies, schemes, corruption, shadowy organizations, and other bad things that are done in secret.

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What are some other forms related to insidious?

  • insidiously (adjective)
  • insidiousness (noun)

What are some synonyms for insidious?

What are some words that often get used in discussing insidious?

How is insidious used in real life?

Insidious is a word that describes something as being sneaky and up to no good.

Try using insidious!

Is insidious used correctly in the following sentence?

She was so cunning and crafty that we didn’t have a clue about her insidious plans until after she had betrayed us.

Example sentences from the Web for insidious

British Dictionary definitions for insidious

/ (ɪnˈsɪdɪəs) /


stealthy, subtle, cunning, or treacherous
working in a subtle or apparently innocuous way, but nevertheless deadlyan insidious illness

Derived forms of insidious

insidiously, adverbinsidiousness, noun

Word Origin for insidious

C16: from Latin insidiōsus cunning, from insidiae an ambush, from insidēre to sit in; see insessorial
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for insidious

[ ĭn-sĭdē-əs ]


Being a disease that progresses with few or no symptoms to indicate its gravity.

Other words from insidious

in•sidi•ous•ly adv.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.