An Etymological Dictionary of the German Language/sauer
sauer, adj., ‘sour, acid, bitter,’ from the equiv. MidHG. and OHG. sûr, adj.; corresponding to Du. zuur, MidLG. and AS. sûr, E. sour, OIc. súrr; Goth. sûrais by chance not recorded. Pre-Teut. *sûró-s is further attested by OSlov. syrŭ, ‘raw,’ and Lith. sū́ras, ‘salty.’ Perhaps Gr. ξὺρός, ‘sour’ (in Hesych.), and the root ἕυ, ‘to scrape, scratch,’ are also allied; in that case sauer would mean ‘scratching.’ From Teut. is derived Fr. sur, ‘sour.’