An Etymological Dictionary of the German Language/Annotated/Eifer
Eifer, masculine, ‘zeal, fervour, passion,’ from late Middle High German îfer, masculine (îfern, neuter), ‘zeal, jealousy.’ The word appeared at a remarkably late period (15th century), and its previous history is quite obscure; it found its way from Upper German, in connection with Luther's translation of the Bible, into Low German, Dutch, Danish and Swedish. Nothing can be adduced in favour of the assumption that the world was borrowed from Upper German eifern. An older German adjective, eifer, ‘sharp, bitter’ (as late as Logan), Old High German eivar, eibar, ‘sharp, bitter,’ Anglo-Saxon âfor, ‘sharp, bitter,’ might perhaps be cognate with Modern High German Eifer.