Impossible to pardon; impossible to excuse or justify(adjective)
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Use "unpardonable" in a sentence
"Constantine University in eastern Algeria, in which he admitted to what he described as unpardonable past crimes by France in Algeria."
"-- ` Well, 'he went on, ` just you look in the Bible, and you'll find that there's what they call the unpardonable sin -- there's no forgiveness for those who've been guilty of it; and if there's truth in that Bible, there's no forgiveness for you, for you've been the biggest blasphemer against the Bible in Crossbourne.'"
"I had paid was the least he could do to repair the result of what he called his unpardonable carelessness."