To lose a game was bad enough, but to round on a comrade was unpardonable; while to lose from the game a half-back of Cameron's calibre was unthinkable.
If you say that someone's behaviour is unpardonable, you mean that it is very wrong or offensive, and completely unacceptable. an unpardonable lack of discipline I must ask a question you may find unpardonable. = unforgivable, inexcusable. unpardonable behaviour is completely unacceptable = unforgivable