Lewis & Short

fācundĭa, ae, f. [facundus], eloquence, fluency (like facundus, not freq. till after the Aug. period; not in Cic., Caes., or Liv.): hic actor tantum poterit a facundia, Ter. Heaut. prol. 13: facundia Graecos, gloria belli Gallos ante Romanos fuisse, Sall. C. 53, 3; so, Graeca, id. J. 63, 3: Graeca Latinaque, Suet. Calig. 20; cf. Quint. 12, 10, 27; Plin. 7, 30, 31, § 117: alere facundiam, Quint. prooem. § 23; Hor. C. 4, 7, 21; Quint. 2, 16, 10; 8, 1, 3; 10, 1, 80 et saep.; Tac. A. 11, 6; Gell. 11, 13, 10; 18, 5, 1; 19, 9, 7 al.
In plur., Gell. 3, 17, 1.
Transf., of a person, Ov. P. 1, 2, 69.