[ in-sid-ee-uhs ]
See synonyms for: insidiousinsidiouslyinsidiousness on

  1. intended to entrap or beguile: an insidious plan.

  2. stealthily treacherous or deceitful: an insidious enemy.

  1. operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect: an insidious disease.

Origin of insidious

First recorded in 1535–45; from Latin insidiōsus “deceitful,” from insidi(ae) (plural) “an ambush” (from insidēre “to sit in”) + -ōsus -ous;see also sit1

Other words for insidious

Other words from insidious

  • 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

Words that may be confused with insidious Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use insidious in a sentence

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

    The Gay Cockade | Temple Bailey
  • 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
  • Indeed, it is perhaps more damaging in its secondary effects because of its insidiousness.

    Honest Money | Arthur Isaac Fonda
  • But this lent a new insidiousness to his temptation, since her contempt would be a refuge from his own.

    The Touchstone | Edith Wharton
  • Its insidiousness makes it exceedingly difficult to recognize.

British Dictionary definitions for insidious


/ (ɪnˈsɪdɪəs) /

  1. stealthy, subtle, cunning, or treacherous

  2. working in a subtle or apparently innocuous way, but nevertheless deadly: an insidious illness

Origin of insidious

C16: from Latin insidiōsus cunning, from insidiae an ambush, from insidēre to sit in; see insessorial

Derived forms of insidious

  • insidiously, adverb
  • insidiousness, noun

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