not appropriately loyal or faithful, dishonest or insincere especially by not respecting one's duties, obligations, or allegiances;
e.g. I would be an unfaithful friend if I revealed his secrets.
(adjective)
engaging in an intimate or sexual relationship with someone other than one's regular partner in a way that violates an understanding or promise (especially marriage vows);
e.g. He was unfaithful to his wife, so she divorced him.
(adjective)
untrustworthy or inaccurate, not faithful to the truth;
e.g. He gave an unfaithful account of the incident, which makes me think he is lying about other things too.