köken: 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…
aile: im Eifer des Gefechts, Glaubenseifer, Übereifer, Wetteifer
köken: Proto-Finnic *inta + -o (compare dialectal inta, Estonian ind, Livonian ind), probably borrowed from Proto-Germanic [Term?] (compare Old Swedish inna (“achievement, accomplishment”)). Alternatively, a Uralic origin has been proposed. Compare Komi-Zyrian ӧд (öd, “speed”).