From Proto-Germanic *arô, from Proto-Indo-European *h₃er-. Cognate with Old English earn, Old High German aro, Old Norse ǫrn.
𐌰𐍂𐌰 (ara) m
This noun followed a declension similar to 𐌰𐌱𐌰 (aba) in Proto-Germanic, so it is likely at least some of the forms had a stem *𐌰𐍂𐌽- (arn-). Those forms are not attested however, only the nominative plural is known with certainty.