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):

insidious
    adj 1: beguiling but harmful; "insidious pleasures"
    2: intended to entrap
    3: working or spreading in a hidden and usually injurious way;
       "glaucoma is an insidious disease"; "a subtle poison" [syn:
       insidious, pernicious, subtle]

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "insidious":
   Byzantine, Machiavellian, Machiavellic, acute, amoral, arch,
   artful, astute, cagey, calculating, canny, chiseling, clever,
   collusive, conscienceless, corrupt, corrupted, covinous, crafty,
   criminal, crooked, cunning, cute, dangerous, dark, deceitful, deep,
   deep-laid, designing, devious, diplomatic, dishonest, dishonorable,
   doubtful, dubious, evasive, false, falsehearted, feline, felonious,
   finagling, fishy, foxy, fraudulent, furtive, gradual, guileful,
   ill-got, ill-gotten, immoral, indirect, ingenious, inventive,
   knowing, not kosher, pawky, perfidious, perilous, politic,
   questionable, ready, resourceful, rotten, scheming, serpentine,
   shady, shameless, sharp, shifty, shrewd, sinister, slick, slippery,
   sly, smooth, snaky, sneaky, sophistical, stealthy, strategic,
   subtile, subtle, supple, surreptitious, suspicious, tactical,
   treacherous, trickish, tricksy, tricky, two-faced, unconscienced,
   unconscientious, unconscionable, underhand, underhanded, unethical,
   unprincipled, unsavory, unscrupulous, unstraightforward, vulpine,
   wary, wily, without remorse, without shame