Refers to a lack of good faith, trustworthiness, or sincerity, often in moral or interpersonal contexts.
彼の不信義な行動に失望した。
I was disappointed by his faithless behavior.
Compound of 不 (fu-, 'not') + 信義 (shingi, 'faith, fidelity'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning of faithlessness.