Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
inter-mestris, e, adj. [mensis], between two months: a mensibus intermestris dictus, Varr. L. L. 6, 2, § 10 Müll.: luna, the new moon, Cat. R. R. 37; Plin. 16, 39, 75, § 194.