Lewis & Short

Mĕnăpĭi, ōrum, m. plur. (Mĕnapis for Menapiis, Mart. 13, 54), a people of Belgic Gaul, between the Meuse and the Scheldt, Caes. B. G. 2, 4, 9; 4, 4, 2; Plin. 4, 17, 31, § 106; Tac. H. 4, 28; Inscr. Orell. 749.
Hence,

  1. A. Mĕnăpĭa, ae, f., the chief town of the Menapii, Aur. Vict. Caes. 39.
  2. B. Mĕnăpĭcus, a, um, adj., of or pertaining to the Menapii, Edict. Diocl. p. 14.