Lewis & Short

Hercynĭa silva, = Ἑρκύνιος δρυμός, the Hercynian Forest, in ancient Germany, sixty days’ journey in length and nine in width, extending from the Schwarzwald, or Black Forest, on the north-east, to the Harz, Caes. B. G. 6, 24 sq.; Mel. 3, 3, 3; Tac. G. 28.
Called also: Hercynius saltus, Plin. 4, 12, 25, § 80; 10, 47, 67, § 132; Tac. G. 30; Liv. 5, 34, 4; and: Hercynium jugum, Plin. 4, 14, 28, § 100.
Also absol.: Hercynia, ae, f., Tac. A. 2, 45 fin.