An Etymological Dictionary of the German Language/Annotated/so

by Friedrich Kluge

so, adverb, ‘thus, so,’ from Middle High German and Old High German sô; corresponding to Old Saxon sô, which seems to represent *swô, although its relation to Anglo-Saxon swâ (English so) and Gothic swa, ‘thus,’ cannot be accurately ascertained. Compare als, also, and solch. The early history of this pronominal adverb (‘in this way’) is obscure; the fact that it assumed the function of a relative (i.e., was used as a conjunction) corresponds to a similar change in the use of der; sô appears as a relative particle in Middle High German, but rarely in Old High German.