adjective
a word that describes or modifies a noun or pronoun, in this case, 'undutiful' describes someone who fails to fulfill their duties or obligations
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The undutiful son refused to help his elderly parents with their daily chores.
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She was labeled as undutiful for not attending her best friend's wedding.
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He's always been an undutiful brat, never following any rules.
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The wilting flower was described as undutiful to its purpose of blooming beautifully.