An Etymological Dictionary of the German Language/Eifer
Eifer, m., ‘zeal, fervour, passion,’ from late MidHG. îfer, m. (îfern, n.), ‘zeal, jealousy.’ The word appeared at a remarkably late period (15th cent.), and its previous history is quite obscure; it found its way from UpG., in connection with Luther's translation of the Bible, into LG., Du., Dan. and Swed. Nothing can be adduced in favour of the assumption that the world was borrowed from UpG. eifern. An older Ger. adj., eifer, ‘sharp, bitter’ (as late as Logan), OHG. eivar, eibar, ‘sharp, bitter,’ AS. âfor, ‘sharp, bitter,’ might perhaps be cognate with ModHG. Eifer.