The plain north of Nargothrond translated “Guarded Plain” (S/168), a combination of talath “plain” and the lenited form of tirnen “guarded” (SA/talath, Ety/TIR).
Conceptual Development: In Silmarillion drafts from the 1930s, this name first appeared as N. Dalath Dirnen (LR/299), also appearing in The Etymologies under the root ᴹ√TIR (Ety/TIR). This form reappeared in Silmarillion revisions from the 1950s-60s, but was revised to Talath Dirnen (WJ/140).
talath (“flat surface, plain”), tirnen (p.p. of tir- “watch, guard, gaze, look at”)