Unto

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Middle English: from until, with to replacing till1 (in its northern dialect meaning ‘to’).

wiktionary

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From Middle English unto, from Old English *untō, *und tō, equivalent to un-(“against; toward; up to”) +‎ to. Cognate with Old Frisian ont to("until"; > Saterland Frisian antou(“until”)) (cf. Old Frisian und(“up to; till”), Old Frisian til(“till; to”)), Old Saxon untō, untuo(“until”), Old High German unze, unzi, unza(“until”), Old Norse und(“as far as; up to”), Gothic 𐌿𐌽𐍄𐌴( untē, “until; as long as”).

etymonline

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unto (prep.)

mid-13c., perhaps a modification of until, with southern to in place of northern equivalent till. Or perhaps a contraction of native *und to, formed on the model of until from Old English *un- "up to, as far as," cognate of the first element in until. "Very rare in standard writers of the 18th c.," according to OED, and since then chiefly in dignified, archaic, or Biblical styles.