in·sid·i·ous /ɪnˈsɪdiəs/ Pronunciation Key - Show Spelled Pronunciation[in-sid-ee-uhs] –adjective 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 eff… Read more

in·sid·i·ous /ɪnˈsɪdiəs/ Pronunciation Key - Show Spelled Pronunciation[in-sid-ee-uhs] –adjective 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.