The word "un-forgivable" refers to something that cannot be forgiven or excused. There are several synonyms for this term, including inexcusable, unforgiving, unpardonable, and indefensible. "Inexcusable" suggests that there is no valid reason or justification for the action or behavior in question. "Unforgiving" implies that the offense is so severe that forgiveness is not an option. "Unpardonable" denotes an offense that cannot be absolved or overlooked. "Indefensible" suggests that the action or behavior is morally wrong and cannot be defended. All these synonyms convey a sense of severity and seriousness, indicating that the transgression is very serious and cannot be easily forgotten or overlooked.