Lewis & Short

Bessi, ōrum, m., = Βέσσοι, Strab.; Βησσοί, Herod., a savage and marauding people in the north-eastern part of Thrace, about the Hœmus mountains, and in the vicinity of the Hebrus, Veg. Mil. 2, 11; 4, 24; Plin. 4, 11, 18, § 40; Cic. Pis. 34, 84; Ov. Tr. 3, 10, 5; 4, 1, 67; Suet. Aug. 3; Isid. Orig. 9, 2, 91.
Sing.: Bessus, i, m., Inscr. Orell. 3548; 3552.
Hence, Bessĭcus, a, um, adj., of the Bessi: gens, Cic. Pis. 34, 84.