If someone is faithless, you cannot trust them to stay true. They break a promise, a bond, or a loyalty. The word is old-fashioned and heavy. You hear it most about love: a faithless husband or wife has gone to someone else. You can also use it for a friend or an ally who switches sides. It is stronger and more literary than unfaithful, which is the everyday word for cheating. There is another, older reading: faithless as “having no religion.” You will meet that in poems and old books. In modern talk, though, if you call a person faithless you are almost always attacking their loyalty, not their church. Though actually, the hurt in the word is the broken faith — the trust that was given and then dropped. It is not a light insult, and it is not the same as “has no confidence.”