as adv., conj., pron.

Old English eallswa "just as, even as, as if, so as, likewise," contraction of eal swa, from all "altogether" + so. Originally an emphatic form of so.

The sense of "wholly so" weakened to "in addition to, in the same way," replacing eke. It was used in Old English to introduce a sequel to a preceding statement, "and so, then, therefore." It was used from c. 1200 in connecting sentences, "in addition, moreover." The compound has parallel forms in German also, Dutch alzoo. English as is a shortened form of it.

Early ME has the phrase as well as the compound. The reduced forms alse, als, as gradually become established in certain constructions, the fuller also in others .... The clear distinction between also and as is not attained until the 15th century. [Middle English Compendium, University of Michigan]

Middle English so, from Old English swa, swæ (adv., conj., pron.) "in this way, in such a manner that," 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 crushed out 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.); it is older as an abbreviation of is it so? (1803). So what as an exclamation of indifference dates from 1934.

The abbreviating phrase and so forth is attested in Old English; and so on is attested from 1724. So far "at such a distance" was in Middle English; so far so good is from 1721, said then to be a Scottish proverb.