Lewis & Short

Mĕsŏpŏtămĭa, ae, f., = Μεσοποταμία, a country of Asia, between the Euphrates and Tigris, now called Al-Jezireh, Cic. N. D. 2, 52, 130; Isid. 13, 21, 10; Mel. 1, 11, 1; Plin. 5, 12, 13, § 66; Sall. H. 4, 51 Dietsch.
Hence, Mĕsŏpŏtămĭus, a, um, adj. (also written Mĕsŏpŏtămēnus), Mesopotamian (post-class.): milites, Val. ap. Vop. Aur. 11: Mesopotameni homines, Sall. H. 4, 53 Dietsch.
In plur. subst.: Mĕsŏpŏ-tămēsii, ōrum, m., the Mesopotamians, Spart. Hadr. 21, 12; Schol. Juv. 1, 104.‡ † mesŏpylus, a, um, adj., = μέσοσπύλη, that is at the middle door, Inscr. Grut. 32, 11.