Unto means to, in old or biblical English. Do unto others, given unto him, unto the ends of the earth.
It is archaic and survives almost entirely in the King James Bible, in hymns and in deliberate old-fashioned style. In modern writing it appears only for effect. The best-known use is the golden rule — do unto others as you would have them do unto you. Note that unto is not more formal than to in modern English, merely older, and that using it in ordinary writing reads as parody.
1prepositionuntoTo, in old or biblical use.
“Do unto others as you would have them do unto you.”
unto is a direction word.
Middle English unto was built from und, an old word meaning as far as, plus to — a doubling for emphasis. It arrived in the thirteenth century, flourished in Tudor English and then stopped almost completely.
Old English
*untō
Middle English
unto
English, today
unto
Middle English unto, from Old English untō, und tō, equivalent to un + to. Cognate with Old Frisian ont to "until"; > stq 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 (“until; as long as”).
Etymology from Wiktionary, CC BY-SA