from The American Heritage® Dictionary of the English Language, 5th Edition.

  • adjective Working or spreading harmfully in a subtle or stealthy manner.
  • adjective Intended to entrap; treacherous.
  • adjective Beguiling but harmful; alluring.

from The Century Dictionary.

  • Lying in wait; hence, deceitful; sly; treacherous.
  • Designed or adapted to entrap; deceptive; insnaring: as, insidious arts.

from the GNU version of the Collaborative International Dictionary of English.

  • adjective Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; -- said of persons.
  • adjective Intended to entrap; characterized by treachery and deceit.
  • adjective Acting or proceeding unobserved or in a seemingly harmless manner, but slowly or eventually doing great damage.
  • adjective (Med.) a disease existing, without marked symptoms, but ready to become active upon some slight occasion; a disease not appearing to be as bad as it really is.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • adjective Producing harm in a stealthy, often gradual, manner.
  • adjective Intending to entrap; alluring but harmful.
  • adjective nonstandard Treacherous.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • adjective beguiling but harmful
  • adjective intended to entrap
  • adjective working or spreading in a hidden and usually injurious way


from The American Heritage® Dictionary of the English Language, 4th Edition

[From Latin īnsidiōsus, from īnsidiae, ambush, from īnsidēre, to sit upon, lie in wait for : in-, in, on; see in– + sedēre, to sit; see sed- in Indo-European roots.]

from Wiktionary, Creative Commons Attribution/Share-Alike License

From Latin īnsidiōsus ("cunning, artful, deceitful"), from īnsidiae ("a lying in wait, an ambush, artifice, stratagem") + -ōsus, from īnsideō ("to sit in or on"), from in ("in, on") + sedeō ("to sit").


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  • Men are so insidious!! Women, can I get an Amen!!

    March 14, 2010

  • Our research has shown that multitasking can have an insidious effect on learning.

    June 9, 2010

  • working in a subtle but destructive way

    Plaque is insidious: we cannot see it, but each day it eats away at our enamel, causing cavities and other dental problems.

    October 19, 2016