Stated to be an alteration of sell , remodelled after ion "son" (OS *jondo). It was "a change assisted by the loss of s in compounds and patronymics", hence the ending -iel in several feminine words
Noldorin
maidh
adjective. pale, fallow, fawn [light yellowish tan colour]
firiel
feminine name. Mortal Maid
eledhwen
feminine name. Elf-maid
firiel
noun. mortal maid
iell
noun. girl, maid
sell
noun. girl, maid (child)
sell Archaic
noun. girl, maid
A “later” name of Lúthien in The Etymologies from the 1930s meaning “Mortal Maid”, marked N[oldorin] and a compound of feir “mortal” with the feminine suffix -iel (Ety/PHIR). In Tolkien’s unfinished story “The Lost Road” from the 1930s, the name Fíriel was given to a Númenórean maiden (LR/62). Later Q. Fíriel was given as a Quenya name.