|part of speech:
||dangerous through cunning, subtlety, and underhandedness.
Their insidious plot included not only the assassination of President Lincoln but the killing of the Vice President and Secretary of State as well.
||seemingly or outwardly harmless, but actually very dangerous.
Glaucoma is an insidious disease that can damage a person's eyes before causing any symptoms.an insidious philosophy
||crooked, cunning, dastardly, devious, foxy, subtle
||insidiously (adv.), insidiousness (n.)