The exact lemma form from which this word derives cannot be reconstructed with any certainty; it may be either *𐌰𐌸𐌽 (*aþn) or *𐌰𐌸𐌽𐍃 (*aþns). It is inherited from the equally uncertain Proto-Germanic *aþna-, which is related to Latin annus, itself proposed to derive from Proto-Italic *atnos, from Proto-Indo-European *h₂ét-no-, probably from *h₂et- (“to go”). Compare the related term 𐌰𐍄𐌰𐌸𐌽𐌹 (ataþni). For the root, compare Sanskrit अतति (atati, “he wanders, goes”). In Italic and Germanic, the root shifted to refer to "year" only (as a "going" of time).
In Latin annus, the nasal geminate can be the product of assimilation of a t by a following n, cf. penna, from Proto-Italic *petnā, from Proto-Indo-European *péth₂r̥ ~ pth₂én- (“feather, wing”), from *peth₂- (“to fly”). Such assimilation allows for the connection with this Gothic term. The accent must have been on the first syllable, because end stress would have triggered Kluge's law, giving Proto-Germanic **atta-.