Unforgivable means so hurtful that it could never be pardoned. An unforgivable remark, unforgivable behaviour, quite unforgivable.
It is a statement about the person wronged rather than about the act: only they can decide, which is why saying something is unforgivable on somebody else's behalf sits badly. It is used loosely for anything from betrayal to a missed birthday.
1adjectiveunforgivableSo hurtful that the person wronged could never pardon it.
“Lying to his best friend about something so serious was unforgivable.”
unforgivable is a quality.
From un- + forgivable.
Etymology from Wiktionary, CC BY-SA