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

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
Related formsin·sid·i·ous·ly, adverbin·sid·i·ous·ness, nounun·in·sid·i·ous, adjectiveun·in·sid·i·ous·ly, adverbun·in·sid·i·ous·ness, noun
Can be confusedinsidious invidious

Synonyms for insidious

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1. corrupting. 2. artful, cunning, wily, subtle, crafty. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for insidious


  1. stealthy, subtle, cunning, or treacherous
  2. working in a subtle or apparently innocuous way, but nevertheless deadlyan insidious illness
Derived Formsinsidiously, 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

Word Origin and History for insidious

1540s, from Middle French insidieux (15c.) or directly from Latin insidiosus "deceitful, cunning, artful," from insidiae (plural) "plot, snare, ambush," from insidere "sit on, occupy," from in- "in" (see in- (2)) + sedere "to sit" (see sedentary). Related: Insidiously; insidiousness.

Online Etymology Dictionary, © 2010 Douglas Harper

insidious in Medicine


  1. Being a disease that progresses with few or no symptoms to indicate its gravity.
Related formsin•sidi•ous•ly adv.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.