GNU Collaborative International Dictionary of English

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  1.       

    Insidious , 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

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