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Definition of insidious : 1. Intended to entrap; characterized by treachery and deceit; as, insidious arts.
2. Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; - said of persons; as, the insidious foe.

Synonyms:

harmful, elusive, damaging, unsafe, pestilent, deadly, hazardous, subtle, injurious, perilous, risky, ruinous, dangerous, seductive, pernicious, baneful, disastrous

false, secret, ensnaring, dishonest, sly, treacherous, stealthy

Usage examples:

• Mrs. Hudson looked at first as if this could be but an insidious device for increasing her distress by the assistance of irony. - "Roderick Hudson", Henry James.
• " But ever prowls th' insidious foe, And listens round the fold" This was too much. - "The Delectable Duchy", Arthur Thomas Quiller-Couch.
• He asked De Vayne to a dessert, and took care that the wines should be of an insidious strength. - "Julian Home", Dean Frederic W. Farrar.