Intended to beguile or entrap, as in an insidious plot; stealthily deceitful or treacherous, as in an insidious foe; proceeding in a seemingly harmless way but actually with dangerous effect, as in an insidious disease.
She was terror-stricken by the revelations that were thus made. What were they? Could they be other than the insidious whispers of the bad angel, who would fain have persuaded the struggling woman, as yet only half his victim, that the outward guise of purity was but a lie, and that, if truth were everywhere to be shown, a scarlet letter would blaze forth on many a bosom besides Hester Prynne’s?
—Nathaniel Hawthorne The Scarlet Letter (1850)