From Old Danish Arus, from Old Norse Áróss, a compound of á (“river”) + óss (“mouth”). The first part of the word is an old genitive of Danish å (“small river”) (from Proto-Indo-European *h₂ekʷeh₂ (“water”), related to Latin aqua (“water”)). The latter part is a (now obsolete) word for "river mouth", os (from Proto-Indo-European *h₁éh₃(o)s (“mouth”), related to Latin os (“mouth”)). The name has…