treacherous (adj.) Look up treacherous at Dictionary.com
early 14c., from Old French trecheros, tricheros "deceitful" (12c.), from trecheor, tricheor "cheat, deceiver, liar, impostor, trickster," agent noun from trechier, trichier "to cheat, trick" (see trick (n.)). Figuratively, of things, from c.1600. Related: Treacherously; treacherousness. Middle English had treacher "deceiver, cheat, traitor."