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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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British Dictionary definitions for insidious


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 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
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insidious 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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