An Etymological Dictionary of the German Language/Annotated/grimm

grimm, adjective, ‘fierce, wrathful, furious,’ from Middle High German grim, grimme, Old High German grim, grimmi, ‘unfriendly, frightful, savage’ (to which Modern High German grimmig, Middle High German grimmec and Old High German grimmig are allied). Corresponding to Old Saxon and Anglo-Saxon grim(mm), English grim, Dutch grimmig, Old Icelandic grimmr, Gothic *grimma-; allied to Modern High German gram, root grem (by gradation gram). —