What does "insidious" mean?

Insidious: 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. -- In*sid"i*ous*ly, adv. -- In*sid"i*ous*ness, n.