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 ] | 
