English

Noun

insidiousness ‎(usually uncountable, plural insidiousnesses)

1. A surreptitious harmfulness, quality of entrapment, or treacherousness; the characteristic of being insidious.
   • 1900, Edith Wharton, The Touchstone, ch. 8,

     This lent a new insidiousness to his temptation, since her contempt would be a refuge from his own.

References

• “insidiousness” in An American Dictionary of the English Language, by Noah Webster, 1828.
• “insidiousness” in Dictionary.com Unabridged, v1.0.1, Lexico Publishing Group, 2006.