insidious

[ in-sid-ee-uhs ]
/ ɪnˈsɪd i əs /

adjective

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

Can be confused

insidious invidious
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for insidiously

British Dictionary definitions for insidiously

insidious

/ (ɪnˈsɪdɪəs) /

adjective

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

Derived Forms

insidiously, 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

Medicine definitions for insidiously

insidious

[ ĭn-sĭdē-əs ]

adj.

Being a disease that progresses with few or no symptoms to indicate its gravity.

Related forms

in•sidi•ous•ly adv.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.