different between unremittable vs unpardonable

English

un- +? remittable

unremittable (not comparable)

un- +? pardonable

unpardonable (comparative more unpardonable, superlative most unpardonable)

impossible to pardon; impossible to excuse or justify
- 1900, Gilbert Parker, An Unpardonable Liar:
  "I can understand your being wicked," she said keenly, "but not your being cowardly. That is and was unpardonable."
- 1912, Ralph Connor, Corporal Cameron of the Northwest Mounted Police:
  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.

Webster, Noah (1828) , “unpardonable”, in An American Dictionary of the English Language

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