An Etymological Dictionary of the German Language/Annotated/Eider
Eider, Eidergans, feminine, ‘eider-duck,’ simply Modern High German from Low German eider; the latter, like English eider, eider-duck, is from Icelandic œ̂þr (genitive œ̂þar), œþekolla, ‘eider-duck’ (Modern Icelandic 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 (Swedish ejder, ejderdun). To the Old Icelandic œ̂þr, Sanscrit âtí-, ‘water-bird,’ may correspond; the latter, it is true, is mostly connected with Ente; compare further Norwegian ȧdder, Swedish (dialectic) ȧda, ‘eider-duck’ (from Old Icelandic *áþr, without mutation).