Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. pē̆trōnĭus, a, um, adj. [1. petra], rock-, crag-, mountain- (very rare): canes, Grat. Cyneg. 202.
2. Pē̆trōnĭus, i, m., name of a Roman gens.
So, esp.: Petronius Arbiter, a Roman knight, a favorite of the emperor Nero, perh. the same person with Petronius, the author of a Latin romance which is preserved in a fragmentary state, Tac. A. 16, 17 sq.
In fem.: Pē̆trōnĭa, ae, the first wife of the emperor Vitellius, Tac. H. 2, 64; Suet. Vit. 6.
Hence,