borrowed from Old Norse Æsir, plural of áss "(pagan) god, member of the Aesir," going back to Germanic *ansu-
Germanic evidence for this etymon outside of Old Norse áss is limited and somewhat problematic. A Gothic form *ans is attested indirectly in Jordanes' Getica, a history of the Goths; Jordanes states that after "a great victory they [the Goths] had won in this region, they thereafter called their leaders, by whose good fortune they seemed to have conquered, not mere men, but demigods, that is Anses" ("…iam proceres suos quorum quasi fortuna vincebant, non puros homines, sed semideos id est 'Anses' vocaverunt"). In West Germanic, the word survives only circumstantially: Germanic *ansu-, "god," was apparently the name given to the rune ᚨ, which originally represented /ã/ (nasal /a/); in the Old English futhark (Runic alphabet) this name was applied to the rune ᚩ, with the value /ō/, ōs thereby being the name of this rune. The rune name os (to be read ōs) is attested in the Old English Rune Poem (from the now destroyed manuscript Cotton Otho B.10, transcribed by George Hickes in his Linguarum veterum septentrionalium thesaurus, 1705), as well in some Continental sources (principally, in a treatment of the futhark in the so-called Salzburg-Vienna Alcuin manuscript, Codex Vindobonensis 795; and in the Abecedarium Nordmannicum, a runic poem in a St. Gall manuscript, Codex Sangallensis 878). An Old English word esa (to be read ēsa) attested once in the metrical charm "For a Sudden Stich" (Wið færstice) has been taken as the genitive plural of ōs. Finally, Germanic *ansu- survives as a first element in personal names—in Old English as ōs- (Ōsweald, Ōswine), in Old High German as ans- (Ansgar), and in Old Norse as ās- (Ásleikr, corresponding to Old English Ōslāc, Old High German Ansleh). If *ansu- goes back to Indo-European *h2ens-, the etymon may be compared to Sanskrit ásura-, Avestan ahura-, "lord, member of a class of deities," and to Sanskrit asu-, "life force." A relationship sometimes proposed with Hittite hassu-, "king," has been rejected.