Adjective
not admitting of pardon
Source: WordNetSuccess, n. The one unpardonable sin against one's fellows. Ambrose Bierce
The most unpardonable privilege that men enjoy is their magnanimity. Germaine Greer
The improver of natural knowledge absolutely refuses to acknowledge authority, as such. For him, skepticism is the highest of duties; blind faith the one unpardonable sin. Thomas Henry Huxley
I am not a fan of people who abuse service staff. In fact, I find it intolerable. It's an unpardonable sin as far as I'm concerned, taking out personal business or some other kind of dissatisfaction on a waiter or busboy. Anthony Bourdain
I am not sure just what the unpardonable sin is, but I believe it is a disposition to evade the payment of small bills. Elbert Hubbard
The children of the white families in town were not permitted to associate with me, because my father was committing the then unpardonable crime, in Southern eyes, of educating negroes. Lee De Forest