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