/ Dictionary / Index I insidious: Meaning and Definition of Find definitions for: in•sid•i•ous Pronunciation: (in-sid'ē-us), [key] — adj. 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. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: insidious (Thesaurus) inside track insight Related Content Daily Word Quiz: penury Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language