adjective. 1  Working or spreading harmfully in a subtle or stealthy manner: insidious rumors; an insidious disease. 2  Intended to entrap; treacherous: insidious misinformation. 3  Beguiling but harmful; alluring: insidious pleasures.he definition of insidious is something or someone who works in a subtle or sly way, or in an intent to trap. An example of insidious is a plan that is meant to scam people out of money.