unforgivable
adj. B2 Upper Intermediate US //ˌənfɔɹˈɡɪvəbəɫ// UK //ʌnfəɡˈɪvəbəl// un·for·giv·able
adj. so bad or hurtful that you cannot forgive it. You use this to describe an action that is impossible to excuse.
adj. describing an action or behaviour so offensive or harmful that it cannot be excused or pardoned.
Being late for your own wedding is unforgivable.
The manager felt that lying about the safety reports was an unforgivable breach of trust.
While minor errors in judgment are expected in high-pressure roles, the deliberate concealment of financial losses was considered an unforgivable failure of leadership by the board.
Usage
Often used predicatively after linking verbs like 'be', 'seem', or 'remain'.