纯朴 and 淳朴 (chúnpǔ) are variant spellings with the same meaning. Both are accepted, though 淳朴 is more common in modern texts. They describe people or customs as genuine and unspoiled.
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Think of 纯 (pure) + 朴 (plain) = someone who is purely plain, without pretense - a genuine, down-to-earth person.
