unfaithful

adj. 1 not faithful, esp. Adulterous. 2 treacherous; disloyal.  unfaithfully adv. Unfaithfulness n.

a. Not faithful; not observant of promises, vows, allegiance, or duty; violating trust or confidence; treacherous; perfidious; as, an unfaithful subject; an unfaithful agent or servant.