to and fro (adv.)

"away, backwards," c. 1200, Northern English and Scottish dialectal fra, Midlands dialect fro, from Old Norse fra "from," from Proto-Germanic *fra "forward, away from," from PIE *pro- (see pro-), extended form of root *per- (1) "forward," hence "in front of, before, toward, near," etc. The Norse word is equivalent to Old English fram, thus fro is a doublet of from.

Old English to, ta, te, "in the direction of, as far as (a place, state, goal)," opposite of from; also "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"). Not found in Scandinavian, where the equivalent of till (prep.) is used.

This is reconstructed to be 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 -dē "to, toward," Old Irish do, Lithuanian da-), from demonstrative *de-. Also see too.

English to also supplies the place of the dative in other languages. The near-universal use of to as the verbal particle with infinitives (to sleep, to dream, etc.) arose in Middle English out of the Old English dative use of to and helped shade out the Old English inflectional endings. In this use to is a mere sign, without meaning. Compare similar use of German zu, French à, de.

As an adverb of motion, direction, etc., "to a place in view, to a thing to be done," in Old English. This use was frequent in Middle English in verbal combinations where it renders Latin ad-, com-, con-, ex-, in-, ob-. As a conjunction, "until, up to the time that," by late Old English.

The distribution of verbs among at, to, with, of has been idiosyncratic and varied. Before vowels it was sometimes shortened to t'. The 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]

Used absolutely at the end of a clause. with ellipsis of infinitive (same as the proceeding clause: would do it but don't have time to), it is attested from 14c.; OED reports it "rare before 19th c.; now a frequent colloquialism."