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[in-sid-ee-uh s] /ɪnˈsɪd i əs/
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 forms
insidiously, adverb
insidiousness, noun
uninsidious, adjective
uninsidiously, adverb
uninsidiousness, noun
Can be confused
insidious, invidious.
1. corrupting. 2. artful, cunning, wily, subtle, crafty. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2017.
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Examples from the Web for insidiousness
Historical Examples
  • The constancy and the strength of temptation, and the insidiousness of the arguments it was supported by, has been proverbial.

    Is Life Worth Living? William Hurrell Mallock
  • We suspected not that deceit, insidiousness, and slavery were to be found beneath the sun.

    Imogen William Godwin
  • She saw O-liver start from his chair and sink back, helpless against the insidiousness of this attack.

    The Gay Cockade Temple Bailey
  • Its insidiousness makes it exceedingly difficult to recognize.

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

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

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

    Christmas Penny Readings George Manville Fenn
  • Indeed, it is perhaps more damaging in its secondary effects because of its insidiousness.

    Honest Money Arthur Isaac Fonda
British Dictionary definitions for insidiousness


stealthy, subtle, cunning, or treacherous
working in a subtle or apparently innocuous way, but nevertheless deadly: an insidious illness
Derived Forms
insidiously, adverb
insidiousness, noun
Word Origin
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
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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
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insidiousness in Medicine

insidious in·sid·i·ous (ĭn-sĭd'ē-əs)
Being a disease that progresses with few or no symptoms to indicate its gravity.

in·sid'i·ous·ly adv.
in·sid'i·ous·ness n.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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