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.]
Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; -- said of persons; as, the insidious foe.
Intended to entrap; characterized by treachery and deceit; as, insidious arts.
The insidious whisper of the bad angel.
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.
© Webster 1913.