*austrą^²

Adverbialized accusative singular of the *austra- formation represented by the adjective *austraz, originally ‘to the east, eastward’. EWA reconstructs an old accusative *austran < pre-Germanic *aus-tro-m, rather than an endingless locative **austre (Lühr 2017: s.v. ôstar). See *austraz for the deeper etymology.