[ in-sid-ee-uhs ]
/ 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.

Nearby words

  1. inside track, the,
  2. inside-out,
  3. insider,
  4. insider dealing,
  5. insider trading,
  6. insidiously,
  7. insight,
  8. insightful,
  9. insightfully,
  10. insigne

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
Can be confusedinsidious invidious Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for insidious

British Dictionary definitions for insidious


/ (ɪnˈsɪdɪəs) /


stealthy, subtle, cunning, or treacherous
working in a subtle or apparently innocuous way, but nevertheless deadlyan insidious illness
Derived Formsinsidiously, adverbinsidiousness, noun

Word Origin for insidious

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

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

Medicine definitions for insidious


[ ĭn-sĭdē-əs ]


Being a disease that progresses with few or no symptoms to indicate its gravity.
Related formsin•sidi•ous•ly adv.

The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.