An Etymological Dictionary of the German Language/Grat
Grat, m., and Gräte, f., ‘point, ridge, fish-bone,’ from MidHG. grât, m., ‘fish-bone, awn, back-bone, mountain ridge’; in ModHG. the word has assumed two forms, according to the meanings. Since Graune, ‘awn,’ has also the dial. sense ‘fish-bone,’ both words may perhaps be traced back to a common root gra-, ‘to be pointed, bristly.’