'Insidiousness' definitions:

Definition of 'insidiousness'

noun

Subtle and cumulative harmfulness (especially of a disease)

noun

The quality of being designed to entrap

From: GCIDE

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]

From: GCIDE

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]