[1] : The Collaborative International Dictionary of English v.0.48

insidious \insid"ious\, 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. --
        insid"iously, adv. -- insid"iousness, n.
        1913 webster
see also:
sit insidious disease insid"iously insid"iousness 

[2] : WordNet (r) 2.0

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: pernicious, subtle
see also:
pernicious subtle 

[3] : 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

Insidious >> I Words
Insidious, definition of term: Insidious
insidious_pag1.html

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Wired News: Beware Those Insidious Vcards
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