insiduous

Adjective

insidious (comparative more insidious, superlative most insidious)

1. Causing harm in a stealthy, often gradual, manner.
   • Strong and vigorous man as he looks, Livingstone has been for years the victim of a secret and insidious disease.1847, George Lippard, The Quaker City: or, The monks of Monk-Hall

   Synonyms:

   subtle

2. Intending to entrap; alluring but harmful.
   • Hansel and Gretel were lured by the witch’s insidious gingerbread house.
3. (nonstandard) Treacherous.
   • The battle was lost due to the actions of insidious defectors.

   Synonyms:

   perfidious, proditorious, traitorous, booky, dastardly, disleal, disloyal, double-crossing, double-dealing, double-faced