Eifer.

From Middle High German *īver (“jealousy”). The simplex is first attested in 1494 as yfer, but in 1349 the derivative eifrær (in a southern text that already reflects diphthongisation, thus equivalent to a classical *īverære). An Upper German dialect word, which was chosen by Luther to express the idea of Ancient Greek ζῆλος (zêlos). From the German then Dutch ijver, Middle Low German īver, and further Danish iver etc. Further origin unknown. Perhaps an ablaut variant of Old High German eibar, eivar (“sharp, bitter”), from Proto-West Germanic *aibr, from Proto-Germanic *aibraz, whence also Old English āfor.