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 (source also of Old Saxon, Middle Dutch, Old High German so, Old Norse sva, Danish saa, Swedish så, Old Frisian sa, Dutch zo, German so "so," Gothic swa "as"), from PIE reflexive pronominal stem *swo- "so" (source also of 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 a word confirming a previous statement, late Old English; also from late Old English as an intensive in an affirmative clause (such as so very "exceedingly, extremely"). 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. Abbreviating phrase and so on is attested from 1724. So far so good is from 1721.