faithless
adj. C1 Advanced UK //fˈeɪθləs// faith·less
adj. not being loyal or honest to someone or something you have promised to support. It describes a person who breaks their word or a relationship.
adj. lacking in faith or loyalty; disloyal or untrustworthy. Often used to describe a person who breaks a vow or a political party that fails to support its own candidate.
He felt betrayed by his faithless friend.
The politician was criticized for his faithless behavior, as he had promised to support the bill but voted against it.
The novel explores the emotional devastation of a faithless marriage, where the protagonist must navigate the ruins of a relationship built on a foundation of broken promises.
From Middle English feithles, equivalent to faith + -less.