"soft feathers of the eider-duck" (such as it uses to line its nest), 1774; see eider + down (n.1). Ultimately from Icelandic æðardun, via a Scandinavian source (compare Danish ederdunn) or German Eiderdon.
type of duck, 1743, from German Eider or Dutch eider, both from Old Norse æþar, genitive of æþr "duck," according to Watkins from a North Germanic root *athi-, from Proto-Germanic *ethi-, from PIE "probable root" *eti- "eider duck."
"first feathers of a baby bird; soft covering of fowls under the feathers, the under-plumage of birds," used for stuffing pillows and feather-beds, mid-14c., from Old Norse dunn, which is of uncertain origin. Extended in Modern English to the soft hair of the human face and fine soft pubescence upon plants and some fruit.
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<a href="https://www.etymonline.com/word/eiderdown">Etymology of eiderdown by etymonline</a>
Harper, D. (n.d.). Etymology of eiderdown. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/eiderdown