Co je unfaithful? Co je unfaithful? Not adhering to promises, obligations, or allegiances; disloyal. See Synonyms at faithless. Breaking trust in a marriage or relationship by having sexual relations with someone other than one's spouse or sexual partner.Not justly representing or reflecting the facts or a primary source; inaccurate: an account that was unfaithful to what really happened; a translation that was unfaithful to the original.