This word appeared as rhaw(f) in notes from the late 1960s with Quenya cognates Q. hráva “wild” and Q. hravan “wild beast”, all derived from the root √S-RAB “wild” (PE17/78). In this document rhaw(f) served as the basis for Rhovanion “Wilderland”. In notes from 1969, rhaw (< rhawf) reappeared with the gloss “wild beast” and a peculiar plural form i·rhaw (PE23/139). Elsewhere Tolkien said words with the diphthong au became oe in the plural (PE17/25), such as noeg plural of naug “dwarf” (UT/100). The loss of final f [v] after the u-diphthong au is the usual Sindarin development.

Conceptual Development: N. rhaw appeared in The Etymologies of the 1930s derived from primitive ᴹ✶rāba “wild, untamed” under the root ᴹ√RAB (Ety/RAB). Christopher Tolkien glossed this word as “wilderness” in The Etymologies as published in The Lost Road (LR/382), but in their Addenda and Corrigenda to the Etymologies Carl Hostetter and Patrick Wynne indicate this gloss actually applied to N. rhofan (VT46/10), meaning the adjective likely had the same meaning as its primitive form: “wild, untamed”. In The Etymologies, the primitive r unvoiced to rh as was typical for Noldorin, and it seems in the 1960s Tolkien revised the root for this word to √S-RAB so that the Sindarin form would still begin with rh.

Neo-Sindarin: For purposes of Neo-Sindarin, I assume this word can be used as both an adjective “wild, untamed” or a noun “wild beast”, but I would assume its plural form is rhoef to be consistent with Sindarin plurals appearing elsewhere, with the final -f preserved in the plural since it no longer followed a u-diphthong.

According to the editorial corrections in EtyAC/RAB, the gloss “wilderness” in The Etymologies is actually associated with a different word, rhofan, so that the proper gloss of this word is likely the same as its Quenya cognate ráva¹: “wild, untamed”.