insidious

adj

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.”

   “At some point in time they may become the source of an insidious cancer.”

   “The nurse always must be alert to signs of slow leak or insidious infiltration.”

   “The impact on rural communities of rail closures was acute, but I would argue that the worst outcome it created was the long-term diminishment of suburban capacity outside London, which has had a far more insidious effect on rail usage nationally.”

2. Intending to entrap; alluring but harmful.

   “Hansel and Gretel were lured by the witch’s insidious gingerbread house.”

   “Gashford slid his cold insidious palm into his master's grasp, and so, hand in hand, and followed still by Barnaby and by his mother too, they mingled with the concourse.”

   “The insidious whispers of the bad angel.”

   “All these facts clearly appear to me now to establish that the sanctioned scheme was a part of a bigger and […] more insidious scheme which was to hoodwink the creditors and to firmly establish and consolidate the position […]”

   “The atmosphere of this insidious city comes out to meet him the moment he touches the European shore; for in London he meets Maria Gostrey just over from France.”

3. (nonstandard)Treacherous.

   “The battle was lost due to the actions of insidious defectors.”

   “But with whom do you contract that alliance? With the natural enemy of France — that insidious house of Austria — which detests our country from feeling, system, and necessity.”

   “‘Believe me,’ he shouted, ‘these insidious folk talk dangerous nonsense. I hear they are spouting out their ridiculous platitudes not five miles from this park in which we are standing…’”