From Old Norse æðr, from Proto-Germanic *ēdrǭ, *ēþrǭ.
æðr f (genitive singular æðrar, plural æðrar)
| f6 | Singular | Plural | ||
| Indefinite | Definite | Indefinite | Definite | |
| Nominative | æðr | æðrin | æðrar | æðrarnar |
| Accusative | æðr | æðrina | æðrar | æðrarnar |
| Dative | æðr | æðrini | æðrum | æðrunum |
| Genitive | æðrar | æðrarinnar | æðra | æðranna |
From Proto-Germanic *ēdrǭ, *ēþrǭ; whence also Old English ǣder, ǣdre, Old High German ādara (German Ader).
æðr f (genitive æðar, dative æði, plural æðar)
In Old Icelandic, the word lost its radical r, it being reinterpreted as a nominative ending, and comes to be nominative æðr, accusative and dative æði, genitive æðar, plural æðar, leading to the modern Icelandic æð, whereas the radical r is preserved in Faroese, as well as in the other Nordic languages.
Uncertain. Cognate to Proto-Samic *(h)āvtë, either from a common unattested language or through mutual loans. Sanskrit आति (āti, “a type of aquatic bird”) has been suggested, but it is inconsistent with either Sami cognates or the unattested masculine form *áðr, both suggesting a Proto-Germanic *aw(V)diz, probably Proto-Germanic *awadiz. Derivations from an ultimate Proto-Indo-European *h₂éwis are considered "unconvincing" by Guus Kroonen, which strengthens the non-Indo-European substrate hypothesis.
According to Watkins, from a North Germanic root (*athi), from Proto-Germanic *ethi-, from a theoretical Proto-Indo-European root *eti- (“eider”).[1]
Compare the difficulty in precising the relationship between Old Norse igða (“small bird”) and Akkala Sami avigʒinĉ (“chickadee”).
æðr f (genitive æðar, dative æði, plural æðar)