c. 1300, "unbelieving," from faith + -less. Meaning "insincere, deceptive" is mid-14c. Related: Faithlessly; faithlessness.
Unfaithful ... especially means a lack of fidelity to trust or duty, a failure to perform what is due, however much may be implied in that. Faithless is negative in form, but positive in sense; the faithless man does something which is a breach of faith; the sleeping sentinel is unfaithful; the deserter is faithless. [Century Dictionary, 1895]