An adjectival form of ang “iron” appearing in several names such as Angrenost “Iron Fortress, Isengard” (LotR/473) and (in its plural form) Ered Engrin “Iron Mountains” (S/118).
Conceptual Development: In the Gnomish Lexicon of the 1910s the adjective was G. angrin “of iron, iron” (GL/19). In the Early Noldorin Dictionary of the 1920s it became ᴱN. angren “of iron”, along with another adjective ᴱN. angeb “like iron” (PE13/159). N. angren “of iron” appeared in The Etymologies of the 1930s under the root ᴹ√ANGĀ (Ety/ANGĀ), and appeared regularly in Tolkien’s writings thereafter.
Sindarin name of the river Isen (UT/318), it is simply the adjective angren “iron” used as a name.