In Samuel E. Martin's "A Reference Grammar of Japanese", in a section (Chapter 23, p. 1041) on putative etymologies for some Japanese interjections like さあ or まあ, he asserts:

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> Although others [other interjections] seem like little more than grunts or shouts — aa, yaa, yoo — they may have originated as shortenings of more legitimate etyma; oo 'yea' is, after all, a Chinese loanword.
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Really? If so, a loan from which Chinese word?