in·sid·i·ous

[in-sid-ee-uhs]
adjective
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:
1535–45; < Latin insidiōsus deceitful, equivalent to insidi(ae) (plural) an ambush (derivative of insidēre to sit in or on) + -ōsus -ous

in·sid·i·ous·ly, adverb
in·sid·i·ous·ness, noun
un·in·sid·i·ous, adjective
un·in·sid·i·ous·ly, adverb
un·in·sid·i·ous·ness, noun

insidious, invidious.


1. corrupting. 2. artful, cunning, wily, subtle, crafty.
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Based on the Random House Dictionary, © Random House, Inc. 2013.
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Insidious is always a great word to know.
So is callithumpian. Does it mean:
an extraordinary or unusual thing, person, or event; an exceptional example or instance.
a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes.
Collins
World English Dictionary
insidious (ɪnˈsɪdɪəs) [Click for IPA pronunciation guide]
 
adj
1.  stealthy, subtle, cunning, or treacherous
2.  working in a subtle or apparently innocuous way, but nevertheless deadly: an insidious illness
 
[C16: from Latin insidiōsus cunning, from insidiae an ambush, from insidēre to sit in; see insessorial]
 
in'sidiously
 
adv
 
in'sidiousness
 
n

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

insidious
1545, from L. insidiosus "deceitful," from insidiæ (pl.) "plot, snare, ambush," from insidere "sit on, occupy," from in- "in" + sedere "to sit" (see sedentary).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

insidious in·sid·i·ous (ĭn-sĭd'ē-əs)
adj.
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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Example sentences
The culprit, some say, may be more insidious-and harder to pin down.
New research indicates that the chemical could have a more insidious side,
  however.
It has also had an insidious effect within the country.
If they multiply at the same rate as opinions, the process can be insidious,
  even destructive.
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