Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. Arbis, is, f., a town in Gedrosia, on the river Arbis, Plin. 6, 23, 26, § 97.
2. Arbis, is, m., a river rising in Carmania, running through Gedrosia, and emptying into the Persian Gulf, Plin. 6, 24, 28, § 109.