— insidiously , adv. — insidiousness , n.
/in sid"ee euhs/ , adj.
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.
[ 1535-45; insidiosus deceitful, equiv. to insidi ( ae ) (pl.) an ambush (deriv. of insidere to SIT in or on) + -osus -OUS ]
Syn. 1. corrupting. 2. artful, cunning, wily, subtle, crafty.