Adjective
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not trustworthy
"an unfaithful reproduction"
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having the character of, or characteristic of, a traitor
"the faithless Benedict Arnold"
"a lying traitorous insurrectionist"
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having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend
"her husband was unfaithful"
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not true to duty or obligation or promises
"an unfaithful lover"