1. Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; said of persons; as, the insidious foe. The insidious witch.
2. Intended to entrap; characterised by treachery and deceit; as, insidious arts. The insidious whisper of the bad angel.
(Science: medicine) (Hawthorne) insidious disease, 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.
Synonym: Crafty, wily, artful, sly, designing, guileful, circumventive, treacherous, deceitful, deceptive.
Origin: L. Insidiosus, fr. Insidiae an ambush, fr. Insidere to sit in; pref. In- _ sedere to sit: cf. F. Insidieux. See Sit.