Acting treacherously hurts or deceives another person, or betrays a secret or promise. A little boy might treacherously give away his sister's hiding place during a game of hide and seek — and his sister, in turn, might treacherously announce to some older kids that he's scared of the dark. In both cases, a secret has been betrayed. Treacherously comes from the adjective treacherous, with its Old French root word, trechier, "to cheat or trick."