The land between river Gelion and the Blue Mountains translated “Land beyond Gelion” (S/124), a combination of thar “across” and the river name Gelion (SA/thar, PE17/34).
Conceptual Development: In Silmarillion drafts from the 1930s, the name first appeared as N. Targelion >> Thargelion (LR/265, 268), and also in The Etymologies with the same derivation as given above (Ety/THAR). In Silmarillion revisions from the 1950s-60s, Tolkien also considered the forms Thargelian (WJ/320) and Thorewilan (WJ/336).
crossing of Gelion; thar (prefix “across”) + Gelion (river name) Thingol - grey cloak; thind (Dor, S “grey, pale”) + coll (“cloak, mantle”); S form of Q Sindikollo; the second element was reinterpreted from earlier gôl “wise” [Etym. THIN-].