from
The Collaborative International Dictionary of English v.0.48
Insidious \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}.]
[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]