A feminine form of Feir “Mortal (Man)”, a combination of that term with the feminine suffix -eth (WJ/387).
Conceptual Development: Earlier words for a “female Mortal” include G. idhwin or idhril from the Gnomish Lexicon of the 1910s (GL/50) and ᴱN. fionwin or fioniel from the Index of Names for The Lay of the Children of Húrin from the early 1920s (PE15/62), all feminized forms of the contemporaneous words for “Mortal” or “Man (as a species)”. The second of these also appeared as ᴱN. fionwin or fionniel in Early Noldorin Word-lists of the 1920s (PE13/143).
fair (“mortal”) + eth (traditional ending for female names)