n. 在不知不觉之间加剧，隐伏（阴险）

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}.]
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.

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.

insidiousness \insidiousness\ n.
A subtle and cumulative harmfulness, especially of a disease.
[WordNet 1.5]

2. the quality of being designed to entrap.
[WordNet 1.5]