Origin and history of herewith

Old English her "in this place, where one puts himself; at this time, toward this place," from Proto-Germanic pronominal stem *hi- (from PIE *ki- "this;" see he) + adverbial suffix -r. Cognate with Old Saxon her, Old Norse, Gothic her, Swedish här, Middle Dutch, Dutch hier, Old High German hiar, German hier.

As the answer to a call, in Old English. Right here "on the spot" is from c. 1200. Here and there "in various places" is from c. 1300. Exact phrase here today and gone tomorrow is by 1680s in writings of Aphra Behn (here to-day, a-wey to-morn is from late 14c.). Here's to _____ as a toast is from 1590s, probably short for here's health to _____. Emphatic this here (adv.) is attested from mid-15c.; colloquially, this here as an adjective is attested from 1762. To be neither here nor there "of no consequence" is attested from 1580s. Here we go again as a sort of verbal rolling of the eyes is attested from 1950.

As a noun, "this place, the present" from c. 1600. Noun phrase here-and-now "this present life" is from 1829.

originally noting opposition, competition, entanglement, but in Middle English shifting to note association or connection; proximity, harmony, alliance.

It is from Old English wið "against, opposite, from;" also "toward, by, near." The sense shifted in Middle English to denote association, combination, and union, partly by influence of Old Norse cognate viðr "against," but also "toward, at," and also perhaps by association with Latin cum "with" (as in pugnare cum "fight with"). Also compare obsolete wine "friend," related to win "to strive, struggle, fight" (see win (v.)) probably on the notion of "comrade-in-arms."

In this sense denoting company or connection it replaced Old English mid, which survives as a prefix (as in midwife; see mid (prep.)). The original sense of "against, in opposition" is retained in compounds such as withhold, withdraw, withstand.

The Old English word is a shortened form related to the first element in widdershins, from Proto-Germanic *withro- "against" (source also of Old Saxon withar "against," Middle Dutch, Dutch weder, Dutch weer "again," Gothic wiþra "against, opposite").

This is reconstructed to be from PIE *wi-tero-, literally "more apart," suffixed form of *wi- "separation" (source also of Sanskrit vi "apart," Avestan vi- "asunder," Sanskrit vitaram "further, farther," Old Church Slavonic vutoru "other, second"). Compare widow (n.).

Phrase with child "pregnant" is recorded from c. 1200. With it "cool, hip, up-to-date" is recorded by 1931 in Black English. What's with? "what are the circumstances of or explanation for" is by 1940.

With and by are so closely allied in many of their uses that it is impossible to lay down a rule by which these uses may at all times be distinguished. The same may be said, but to a less extent, of with and through. [Century Dictionary, 1891]

French avec "with" was originally avoc, from Vulgar Latin *abhoc, from apud hoc, literally "with this."