Lewis & Short

Frīsĭi, ōrum, m., φρίσιοι or φρείσιοι, Ptol. and Dio Cass., in the Middle Ages Frisones and Fresones, the Frisians, a people of northern Germany, between the Rhine and the Ems, in the modern West Friesland, Plin. 4, 15, 27, § 101; Tac. G. 34; id. A. 1, 60; 4, 72 al.
Hence, Frīsĭus, a, um, adj., Frisian, Tac. A. 4, 74.