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."
2. Intended to entrap; characterized by
treachery and deceit; as, insidious arts.
The insidious whisper of the bad
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
Syn. -- Crafty; wily; artful; sly; designing; guileful;
circumventive; treacherous; deceitful; deceptive.
-- In*sid"i*ous*ly, adv. --