The Collaborative International Dictionary of English v.0.48:

Insidious \In*sid"i*ous\, a. [L. insidiosus, fr. insidiae an
   ambush, fr. insidere to sit in; pref. in- + sedere to sit:
   cf. F. insidieux. See Sit.]
   [1913 Webster]
   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.
      [1913 Webster]

   2. Intended to entrap; characterized by treachery and deceit;
      as, insidious arts.
      [1913 Webster]

            The insidious whisper of the bad angel. --Hawthorne.
      [1913 Webster]

   3. Acting or proceeding unobserved or in a seemingly harmless
      manner, but slowly or eventually doing great damage; as,
      an insidious disease; an insidious plot.
      [PJC]

   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.

   Syn: Crafty; wily; artful; sly; designing; guileful;
        circumventive; treacherous; deceitful; deceptive. --
        In*sid"i*ous*ly, adv. -- In*sid"i*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

insidiously
    adv 1: in a harmfully insidious manner; "these drugs act
           insidiously" [syn: insidiously, perniciously]