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Use "unrelentingness" in a sentence
"The fact that he has moved his family from Croydon to Swansea in order to be nominated is proof of his unrelentingness and commitment."
"Godfrey knew all this, and felt it with the greater force because he had constantly suffered annoyance from witnessing his father's sudden fits of unrelentingness, for which his own habitual irresolution deprived him of all sympathy."
"Christianity with the unrelentingness of an Indian."