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.

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

1. corrupting. 2. artful, cunning, wily, subtle, crafty. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for insidiousness

Historical Examples of insidiousness

  • We suspected not that deceit, insidiousness, and slavery were to be found beneath the sun.


    William Godwin

  • Its insidiousness makes it exceedingly difficult to recognize.

  • As to his character: it is too bad to associate him with so much craft and insidiousness.

    Christmas Penny Readings

    George Manville Fenn

  • They believed in the personality, activity and insidiousness of the Devil.


    Sutton E. Griggs

  • The insidiousness of its design has been equalled only by the shameless manner of its being carried into execution.

British Dictionary definitions for insidiousness



stealthy, subtle, cunning, or treacherous
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 insidiousness



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

insidiousness in Medicine




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.