Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
septem-mestris (septĭmestris), e, adj. [mensis], of seven months (late Lat.): infans, Censor. de Die Nat. 8, § 10: septimestris partus, id. ib. 11, § 2.
septĭmestris, v. septemmestris.