insidious

Definitions:

Lying in wait; watching an opportunity to ensnare; treacherous; deceitful; intended to ensnare; working secretly.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (adjective) (adj.) (adjective)
Deceitful; sly; treacherous; intended to ensnare or entrap.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (adjective) (adj.) (adjective)
Treacherous; ensnaring.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (adjective) (adj.) (adjective)
Watching an opportunity to insnare: intended to entrap: treacherous.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (adjective) (adj.) (adjective)
INSIDIOUSNESS.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
Treacherous; deceitful; operating secretly.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
Insidiously.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adverb) (adv.) (adverb)

Usage examples:

I was then too young to detect, as I should now do, the insidious approach of that foe to human life, consumption.
- The Path of Duty, and Other Stories by H. S. Caswell
It was weakening now and the sweat broke out in heavy drops on his forehead as he strove to crush an insidious inward voice that bade him forget the past and take what was his.
- The Shadow of the East by E. M. Hull
Of this offer he accepted; but when this man began to ask some insidious questions, he cast on him a look of contempt and never spoke to him more.
- The Project Gutenberg Memoirs of Napoleon Bonaparte by Bourrienne, Constant, and Stewarton