Lewis & Short

Atrĕbătes, um, m., = Ἀτρέβατοι Strab., Ἀτρεβάτιοι Ptolem., a people in Gallia Belgica, now Artois or Dép. du Pas de Calais, Caes. B. G. 2, 4; 2, 16; 2, 23; 7, 75; Plin. 4, 17, 31, § 106.
In sing.: Atrĕ-bas, ătis, m., an Atrebatian, Caes. B. C. 4, 35.
Hence, Atrĕbătĭcus, a, um, adj., Atrebatian: sagum, Treb. Gall. 6.