- so O.E. swa, swæ "in this way," from P.Gmc. *swa (Cf. O.S., M.Du., O.H.G. so, O.N. sva, Dan. saa, Swed. sô, O.Fris. sa, Du. zo, Ger. so "so," Goth. swa "as"), from PIE reflexive pronomial stem *s(w)o- (Cf. Gk. hos "as," O.Latin suad "so," L. se "himself"). So? as a term of dismissal is attested from 1886 (short for is that so?); so what as an exclamation of indifference dates from 1934. So-so "mediocre" is from 1520s; so-and-so is from 1596 meaning "something unspecified;" first recorded 1897 as a euphemistic term of abuse.The adverb so at the beginning of a sentence ('So I'll pay for it!'), probably of Yiddish origin, occurs frequently in conversation. [M.Pei, "Story of English," 1952]
Etymology dictionary. 2014.