Faithless means not keeping faith or promises. A faithless friend, faithless to her, faithless promises.
It means breaking trust rather than having no religion, though the second sense exists and is older. It is a heavy and slightly archaic charge. In American politics, a faithless elector is one who votes against the pledge, which is a defined and rare event. Note that the compound is Middle English, built from faith plus -less exactly as it still is.
1adjectivefaithlessNot keeping faith or promises.
“Her faithless husband had been secretly seeing another woman for years.”
synonyms: treacherous, disloyal
faithless is a quality of disloyalty.
Middle English feithles is faith plus -less, from Latin fides, trust. Fidelity, confide and defy come from the same trust.
Middle English
feithles
English, today
faithless
From Middle English feithles, equivalent to faith + less.
Etymology from Wiktionary, CC BY-SA