Um

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natural utterance: first recorded in English in the early 17th century.

wiktionary

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Onomatopoeic. 

From Middle English um, from Old Norse um, umb(“around, about”), from Proto-Germanic *umbi(“around”), from Proto-Indo-European *h₂m̥bʰi(“round about, around”). Cognate with Old English ymbe(“around”), West Frisian om(“around”), Dutch om(“around”), German um(“around”). More at umbe.

um

etymonline

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um

a sound denoting hesitation, 1670s.