un·faith·ful 
(ŭn-fāthfəl)
(ŭn-fāthfəl)adj.
1.
a. Not adhering to promises, obligations, or allegiances; disloyal. See Synonyms at faithless.
b. Breaking trust in a marriage or relationship by having sexual relations with someone other than one's spouse or sexual partner.
2. 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.
un·faithful·ly adv.
un·faithful·ness n.