An Etymological Dictionary of the German Language/Eider
Eider, Eidergans, f., ‘eider-duck,’ simply ModHG. from LG. eider; the latter, like E. eider, eider-duck, is from Ic. œ̂þr (gen. œ̂þar), œþekolla, ‘eider-duck’ (Mod. Ic. œ is pronounced like ei). Eider-down was brought by the Hanse traders from Iceland to England and Germany, and from the latter imported into Sweden (Swed. ejder, ejderdun). To the OIc. œ̂þr, Sans. âtí-, ‘water-bird,’ may correspond; the latter, it is true, is mostly connected with Ente; comp. further Norw. ȧdder, Swed. (dial.) ȧda, ‘eider-duck’ (from OIc. *áþr, without mutation).