From Longman Dictionary of Contemporary Englishunpardonableun‧par‧don‧a‧ble /ʌnˈpɑːdnəbəl $ -ˈpɑːr-/ adjective formal FORGIVE#unpardonable behaviour is completely unacceptable SYN unforgivable an unpardonable offence —unpardonably adverb
Examples from the Corpus
unpardonable• That would be seen by many as an unpardonable failure that disqualifies him from having a second run.• To some it is even the ultimate treason against faith, the unpardonable sin. 