"purpose," O.E. sacu "a cause at law, crime, dispute, guilt," from P.Gmc. *sako "affair, thing, charge, accusation" (cf. O.N. sök "charge, lawsuit, effect, cause," O.Fris. seke "strife, dispute, matter, thing," Du. zaak, Ger. sache "thing, matter, affair, cause"), from PIE base *sag- "to investigate"
(cf. O.E. secan, Goth. sokjan "to seek;" see seek
). Much of the word's original meaning has been taken over by case, cause, and it survives largely in phrases for the sake of (early 13c.) and for _______'s sake (c.1300, originally for God's sake), both probably are from O.N., as these forms have not been found in O.E.