Etymology
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unfaithful (adj.)

mid-14c., "acting falsely," from un- (1) "not" + faithful. In Middle English it also had a sense of "infidel, unbelieving, irreligious" (late 14c.). Sense of "not faithful in marriage" is attested from 1828. Related: Unfaithfully; unfaithfulness.

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Definitions of unfaithful

unfaithful (adj.)
not true to duty or obligation or promises;
an unfaithful lover
unfaithful (adj.)
having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend;
her husband was unfaithful
unfaithful (adj.)
having the character of, or characteristic of, a traitor;
unfaithful (adj.)
not trustworthy;
an unfaithful reproduction
From wordnet.princeton.edu