so (adv.) Look up so at Dictionary.com
Old English swa, swæ (adv., conj., pron.) "in this way," also "to that extent; so as, consequently, therefore," and purely intensive; from Proto-Germanic *swa (cognates: Old Saxon, Middle Dutch, Old High German so, Old Norse sva, Danish saa, Swedish , Old Frisian sa, Dutch zo, German so "so," Gothic swa "as"), from PIE reflexive pronomial stem *swo- "so" (cognates: Greek hos "as," Old Latin suad "so," Latin se "himself"), derivative of *s(w)e-, pronoun of the third person and reflexive (see idiom).

Old English swa frequently was strengthened by eall, and so also is contained in compounds as, also, such. The -w- was eliminated by contraction from 12c.; compare two, which underwent the same process but retained its spelling. As an "introductory particle" [OED] from 1590s. Used to add emphasis or contradict a negative from 1913. So in mid-20c. British slang could mean "homosexual" (adj.). So? as a term of dismissal is attested from 1886 (short for is that so?, etc.). So what as an exclamation of indifference dates from 1934. So-and-so is from 1596 meaning "something unspecified;" first recorded 1897 as a euphemistic term of abuse. Abbreviating phrase and so on is attested from 1724. So far so good is from 1721.