Hercynian (adj.)
1580s, a classical term of vague application designating the forest-covered mountains of ancient Germany (especially das Harzgebirge), from Latin hercynia (silva) "Hercynian (forest)," related to Greek herkynios (drymos), probably from Old Celtic *perkunya, from PIE *perq(o)- "oak, oak forest, wooded mountain" (see fir). As a term in geology from 1880.