unfaithful

not true to duty or obligation or promises

• “an unfaithful lover”

having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend

• “her husband was unfaithful”

deliberately and abominably disloyal or likely to betray trust or confidence

• “the faithless Benedict Arnold”
• “a lying traitorous insurrectionist”

not trustworthy

• “an unfaithful reproduction”