unfaithful

adjective ! Definition from WordNet 3.1, Princeton University. More about unfaithful at Wordnik → Where our words come from →

1. Not true to duty or obligation or promises.

   an unfaithful lover

2. Having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend.

   her husband was unfaithful

3. Having the character of, or characteristic of, a traitor.

   the faithless Benedict Arnold