Insidious

Part of speech: adjective (a.)

1. Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; -- said of persons; as, the insidious foe. "The insidious witch." Cowper.

2. Intended to entrap; characterized by treachery and deceit; as, insidious arts. The insidious whisper of the bad angel. Hawthorne. Insidious disease (Med.), a disease existing, without marked symptoms, but ready to become active upon some slight occasion; a disease not appearing to be as bad as it really is.

Synonyms: Crafty; wily; artful; sly; designing; guileful; circumventive; treacherous; deceitful; deceptive. In*sid"i*ous*ly, adv. In*sid"i*ous*ness, n.