Old English wið "against, opposite, toward," a shortened form related to wiðer, from Proto-Germanic *withro- "against" (cf. Old Saxon withar "against," Old Norse viðr "against, with, toward, at," Middle Dutch, Dutch weder, Dutch weer "again," Gothic wiþra "against, opposite"), from PIE *wi-tero-, literally "more apart," from root *wi- "separation" (cf. Sanskrit vi, Avestan vi- "asunder," Sanskrit vitaram "further, farther," Old Church Slavonic vutoru "other, second").
Sense shifted in Middle English to denote association, combination, and union, partly by influence of Old Norse vidh, and also perhaps by Latin cum "with" (as in pugnare cum "fight with"). In this sense, it replaced Old English mid "with," which survives only as a prefix (e.g. midwife). Original sense of "against, in opposition" is retained in compounds such as withhold, withdraw, withstand. Often treated as a conjunction by ungrammatical writers and used where and would be correct. First record of with child "pregnant" is recorded from c.1200. With it "cool" is black slang, recorded by 1931. French avec "with" was originally avoc, from Vulgar Latin *abhoc, from apud hoc, literally "with this."
Old English hit, neuter nominative and accusative of third person singular pronoun, from Proto-Germanic demonstrative base *khi- (cf. Old Frisian hit, Dutch het, Gothic hita "it"), from PIE *ko- "this" (see he). Used in place of any neuter noun, hence, as gender faded in Middle English, it took on the meaning "thing or animal spoken about before."
The h- was lost due to being in an unemphasized position, as in modern speech the h- in "give it to him," "ask her," "is only heard in the careful speech of the partially educated" [Weekley]. It "the sex act" is from 1610s; meaning "sex appeal (especially in a woman)" first attested 1904 in works of Rudyard Kipling, popularized 1927 as title of a book by Elinor Glyn, and by application of It Girl to silent-film star Clara Bow (1905-1965). In children's games, meaning "the one who must tag the others" is attested from 1842.
Having the usual accompaniment, that is, onions with hamburger, cream with coffee, etc: We ordered two coffees with, to go (1930s+ Lunch counter)
: Clara Bow, the original it girl