Adjective

1. Lacking faith; lacking belief in something.
   • 1970, Margaret Wade Campbell Deland, Old Chester Days, p. 58:

     "You're so faithless about pills," he said, "that I'm not going to give you any."

2. Not believing in God, religion, or a comparable ideology.
3. Unfaithful; not of true fidelity; inconstant, as a husband or a wife.
   • 1996, Frederick Ahl, Hanna Roisman, The Odyssey re-formed, p. 283:

     Menelaus, who fought to recover his faithless wife, has clearly rooted himself in Sparta for the remainder of his life...

4. Not observant of promises or covenants.
5. Not true to allegiance, duty, or vows; perfidious; treacherous; disloyal.
6. Serving to disappoint or deceive; delusive; unsatisfying.