An Etymological Dictionary of the German Language/so
so, adv., ‘thus, so,’ from MidHG. and OHG. sô; corresponding to OSax. sô, which seems to represent *swô, although its relation to AS. swâ (E. so) and Goth. swa, ‘thus,’ cannot be accurately ascertained. Comp. als, also, and solch. The early history of this pronom. adv. (‘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 MidHG., but rarely in OHG.