adjective
A word that describes or modifies a noun or pronoun. In this case, 'undeceitful' is an adjective describing someone or something that is not deceitful.
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She was known for her undeceitful nature, always speaking the truth no matter what.
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I appreciate how undeceitful you are, always honest with me.
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I can't stand people who are deceitful, I prefer hanging out with the undeceitful ones.
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His eyes were undeceitful, revealing the truth behind his words.