Source : Webster's Revised Unabridged Dictionary (1913)

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}.]
   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.

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

Source : WordNet®

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