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