Uh

google

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1960s: imitative.

wiktionary

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Onomatopoeia of the natural expression of thought. Compare with  er. 

etymonline

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uh

inarticulate sound, attested from c. 1600; uh-huh, spoken affirmative (often ironic or non-committal) is recorded by 1894; negative uh-uh is attested from 1924.