in·sid·i·ous
[in-sid-ee-uh s]
–adjective1.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.
—Synonyms
1. corrupting. 2. artful, cunning, wily, subtle, crafty.