An Etymological Dictionary of the German Language/Annotated/einander
einander, ‘one another,’ thus even in Middle High German einander, Old High German (in the oblique cases) einander, pronoun, ‘one another’ — a senseless combination of the nominative ein with an oblique case of ander; e.g. Old High German sie sind ein anderen ungelîh, ‘they are unlike one another’ (literally the one to the other), zeinanderen quëdan, ‘to say to one another’ (literally one to the others), for which, however, by a remarkable construction, zeinen einanderen may be used in Old High German —