Lewis & Short

Celtĭbēri, ōrum, m., = Κελτίβηρες, Celtiberians, a people in Middle Spain, which originated (cf. Luc. 4, 10) by a mingling of the Celts with the native-born Iberians, Plin. 3, 3, 4, § 19 sq.; Cic. Tusc. 2, 27, 65; Caes. B. C. 1, 38; Flor. 2, 17, 9; 2, 17, 13; 3, 22, 1 al.
In sing.: Celtĭbēr, ēri, a Celtiberian, Cat. 39, 17 Sillig N. cr.
Hence,

  1. A. Celtĭbēr, ēra, ērum, adj., Celtiberian: terrae, Mart. 12, 18: urbes, Val. Max. 5, 1, 5.
  2. B. Celtĭbērĭa, ae, f., = Κελτιβηρία, the land of the Celtiberians, Celtiberia, Caes. B. C. 1, 61; Cic. Phil. 11, 5, 12; Flor. 4, 2, 28; 4, 2, 87 al.: Celtiberia terra, Cat. 39, 17.
  3. C. Celtĭ-bērĭcus, a, um, adj., Celtiberian: bellum, Liv. 42, 3, 1; Val. Max. 3, 2, 21.
    Adv.: Celtĭbērĭcē, in the Celtiberic language, Plin. 33, 3, 12, § 40 (ex conj. Sillig).