Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Arverni, ōrum, m., = Ἀρβερνοί, Plut.; Ἀρουερνοί, Strabo, a people of Gaul, in the present Auvergne, Caes. B. G. 1, 31; 1, 45; 7, 7; Luc. 1, 427; cf. Mann. Gall. p. 130.
Hence, Arvernus, a, um, adj., Arvernian: Arvernum genus (vini), Plin. 14, 1, 3, § 18.