google
ref
1960s: imitative.
wiktionary
ref
Onomatopoeia of the natural expression of thought. Compare with er.
etymonline
ref
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.
