GNU Collaborative International Dictionary of English

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Found one definition

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
   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
   Intended to entrap; characterized by treachery and deceit; as, insidious arts.
   1913 Webster
   The insidious whisper of the bad angel.
   Hawthorne.
   
   1913 Webster
   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