An Etymological Dictionary of the German Language/Sache
Sache, f., ‘thing, matter, affair, business, case,’ from MidHG. sache, OHG. sahha, f., ‘quarrel, cause of dispute, lawsuit, opportunity, affair, cause, reason'; corresponding to the equiv. OIc. sǫk, f., and OSax. saka, f.; comp. Du. zaak, ‘thing,’ AS. sacu, ‘strife, feud,’ E. sake, Goth. sakjô, f., ‘strife, dispute.’ The cognates are connected with Goth. sakan, ‘to strive, dispute,’ AS. sacan, OSax. sakan, OHG. sahhan, ‘to blame, scold; sue (at law).’ The root sak, ‘to contend, sue (at law),’ is peculiar to Teut. The evolution in meaning is worthy of special notice. The general sense ‘case' is a later development of ‘lawsuit, dispute,’ which has been preserved in ModHG. Sachwalter, ‘attorney, advocate’ (see further Widersacher). Old legal parlance developed the former from the latter.