Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ērētum, i, n., = Ἤρητοϝ, an ancient city of the Sabines, now Grotta Marozza (acc. to G. Long, Rimane), Verg. A. 7, 711; Liv. 3, 26; 29 al.
Hence, Ērētīnus, a, um, adj., of Eretum, Tib. 4, 8, 4.