Words related to unto
The two syllables have the same meaning. Originally also used of persons and places. As a conjunction from c. 1300. Similar formation in Swedish intill, Danish indtil (northern English and Scottish formerly also had intill/intil "into, in"). The Modern German equivalent, bis (Old High German biaz), is a similar compound, of Old High German bi "by, at, to" and zu "to."
Old English to "in the direction of, for the purpose of, furthermore," from West Germanic *to (source also of Old Saxon and Old Frisian to, Dutch toe, Old High German zuo, German zu "to"), from PIE pronominal base *do- "to, toward, upward" (source also of Latin donec "as long as," Old Church Slavonic do "as far as, to," Greek suffix -de "to, toward," Old Irish do, Lithuanian da-), from demonstrative *de-. Not found in Scandinavian, where the equivalent of till (prep.) is used.
The nearly universal use of to with infinitives (to sleep, to dream, etc.) arose in Middle English out of the Old English dative use of to, and it helped drive out the Old English inflectional endings (though in this use to itself is a mere sign, without meaning).
Commonly used as a prefix in Middle English (to-hear "listen to," etc.), but few of these survive (to-do, together, and time references such as today, tonight, tomorrow — Chaucer also has to-yeere). To and fro "side to side" is attested from mid-14c. Phrase what's it to you "how does that concern you?" (1819) is a modern form of an old question:
Huæd is ðec ðæs?
[John xxi:22, in Lindisfarne Gospel, c.950]
"toward and upon; to and in connection with; to the top of," 1580s, on to, from on + to. It appeared much later than parallel into, unto. As a closed compound, onto (on analogy of into), it is recorded from 1715. "The word is regarded by purists as vulgar, and is avoided by careful writers" [Century Dictionary, 1895].