Causing harm in a stealthy, often gradual, manner.
Intending to entrap; alluring but harmful.
Hansel and Gretel were lured by the witch’s insidious gingerbread house.
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 […]
This seemed to her the worst defilement into which this insidious city had cheated her and in her agitation, she nearly ran into the latrine, […]
(nonstandard) Treacherous.
The battle was lost due to the actions of insidious defectors.