Faithlessness is the noun: the fact or quality of being faithless. It is the broken trust itself, not the person. You can speak of someone’s faithlessness in a marriage, or the faithlessness of an ally who walked away. It is a formal, slightly old word — more at home in a novel, a sermon, or a serious essay than in a text message. Everyday English prefers cheating, betrayal, or being unfaithful. The shape is simple: take faithless and add -ness, the ending that turns a describing word into a name for a quality. Though actually, using such a big noun can sound as if you are standing far away from the hurt, wrapping it in a long word. It still points at the same wound as faithless: not “no religion,” but loyalty that did not hold.

不忠；背信。名詞。日常更常說「背叛」「不忠」。