Pronunciation: (in-sid'ē-us), [key]
1. intended to entrap or beguile: an insidious plan.
2. stealthily treacherous or deceitful: an insidious enemy.
3. 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.