Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Sinnĭus, ia, the name of a Roman gens, e. g. Sinnius Capito, a Roman grammarian, Gell. 5, 21, 9.
Hence, Sinnĭānus, a, um, adj., of Sinnius: opinio, Gell. 5, 21, 14.