An adjectival form of ᴹQ. anga “iron” from The Etymologies of the 1930s (Ety/ANGĀ), also mentioned in the Qenya Lexicon and Poetic and Mythological Words of Eldarissa of the 1910s (QL/31; PME/31).
Qenya
iro
adverb. *that is the reason why (relative)
anga
noun. iron
angamanda
place name. Iron Prison, Hells of Iron
angaina
adjective. of iron
ine
pronoun. the one who, the one that
pasta
adjective. smooth
The correlative ᴹQ. iro appeared in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/111), a combination of the relative pronoun ᴹQ. i and ᴹQ. -ro “reason”.
Neo-Quenya: For purposes of Neo-Quenya I would update this to ᴺQ. yaro “that is the reason why” using the relative pronoun ya more commonly used in declined forms in later writings, such as Q. yassë “where [relative]”. For example: istan i saila atan yaro i osto alya (ná) “I know the wise man that is the reason why the city is prosperous”.