Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ămestrătus, i, f., a town on the north coast of Sicily, mentioned only by Cic. and Steph. B., now Mistretta, is prob. the same place as the Amastra of Sil. 14, 267; Cic. Verr. 3, 39, 43; Steph. B. s. v.
Hence, Ămestrătīni, ōrum, m., the inhabitants of Amestratus, Cic. Verr. 3, 39, 89.