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I can't help but be dissatisfied with this answer.
While it explains why 大人 has a non-standard pronunciation, It doesn't really answer the underlying question of where the な came in terms of etymology..
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language.
Join them; it only takes a minute: Sign up As far as I understand, the word 大人 (otona) uses the kanji 大 to represent お and the kanji 人 to represent と.
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