noun, masculine/bo.fʁɛʁ/plural: beaux-frères
brother-in-law (brother of one's wife); brother-in-law (brother of one's husband); brother-in-law (husband of one's sister); brother-in-law (husband of one's brother)
Inherited from Middle French beau frere, beau-frere, beaufrere, beau-frère, beauffrère, biau frere. By surface analysis, beau- + frère.