Friedrich Kluge, An Etymological Dictionary of the German Language (1891)
“ 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. eigen, adjective, ‘own, pertinent, peculiar, odd,’ from the equivalent Modern High German eigen, Old High German eigan; an adjective common to Teutonic; compare Old Saxon êgan, Dutch eigen, Anglo-Saxon âgen, English own, Old Icelandic eiginn; Gothic used swês for *aigans. ”
