An Etymological Dictionary of the German Language/Annotated/Schwager
Schwager, masculine, ‘brother-in-law,’ from Middle High German and Middle Low German swâger, masculine, ‘brother-in-law, father-in-law, son-in-law’ (with regard to the variation in meaning see Neffe); Old High German *swâgar is not recorded; a specifically German word, unknown to the Old Teutonic dialects. Since the word is cognate with Schwaher and Schwieger, and has also an archaic gradation, an Aryan form swêkró-s may be assumed for Schwager (note Old Icelandic svœ́ra, from *swâhrjon, ‘mother-in-law’). See the following word.