Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Marrūcīni (Marūcīni), ōrum, m., a people of Italy, on the Adriatic coast, near the river Aternus, whose chief city was Teate, now Chieti, Cic. Clu. 69, 197; id. Phil. 7, 9, 23; Caes. B. C. 1, 23; Liv. 8, 29; 26, 11 al.; Plin. 3, 12, 17, § 106.
Hence, Marrūcīnus, a, um, adj., of or belonging to the Marrucini, Marrucinian: ager, Liv. 27, 43; Plin. 2, 83, 85, § 199: montes, Stat. S. 4, 4, 86: pubes, Sil. 8, 521: gens, id. 15, 566.
Adv.: Marrūcīnē, in the Marrucine manner, i. e. faithfully, since the faithfulness of the Marrucini was proverbial, Tert. adv. Marc. 5, 17.