Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Trans-rhēnānus, a, um, adj., that is beyond the Rhine, Transrhenish: Germani, Caes. B. G. 5, 2, 5: hostes, i. e. the Batavians, Plin. 19, 1, 2, § 8: lares, Amm. 20, 4, 4.
As subst.: Transrhēnāni, ōrum, m., those living beyond the Rhine, Caes. B. G. 4, 16.