Sir Monier Monier-Williams(1898)A Sanskrit-English dictionary etymologically and philologically arranged with special reference to cognate Indo-European languages, Oxford: Clarendon Press, page 0159
Etymology
From Proto-Indo-European*h₃éh₁os(“mouth”). Cognates include Hittite 𒀀𒄿𒅖(aiš, “mouth”), Latin ōs(“mouth”), Old Church Slavonic оуста(usta, “mouth”) and Old Irish á(“mouth”).