An Etymological Dictionary of the German Language/Schwager
Schwager, m., ‘brother-in-law,’ from MidHG. and MidLG. swâger, m., ‘brother-in-law, father-in-law, son-in-law’ (with regard to the variation in meaning see Neffe); OHG. *swâgar is not recorded; a specifically G. word, unknown to the OTeut. dials. 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 OIc. svœ́ra, from *swâhrjon, ‘mother-in-law’). See the following word.