A fairly common feminine suffix appearing as -il in Sindarin, either formed on its own or as a variant of the feminine suffix -iel. This suffix was also common Noldorin words in The Etymologies of the 1930s, along with an alternate form -ril that seems to be a feminine agental suffix, the equivalent of masculine -(r)on, seen in pairs like N. melethril/melethron “lover” and N. odhril/odhron “parent” (Ety/MEL, ONO). The -il suffix and its -ril variant are seen all the way back in the Gnomish Lexicon of the 1910s in pairs like G. gwadhril/gwadhron “inhabitant” (GL/47) and G. ainil/ainos “god”, female and male respectively (GL/18). So it seems this feminine suffix was well established in Tolkien’s mind.
Primitive elvish
wel
root. *boil, bubble
-iel
suffix. feminine suffix
-ye Reconstructed
suffix. feminine suffix
The most common Quenya suffix for “daughter of” such as in Elerondiel “✱Daughter of Elrond” (PE17/56) or Uinéniel “Daughter of Uinen” (UT/182).
Conceptual Development: The earliest hint of this suffix was ᴱQ. -il mentioned by Tolkien in the Qenya Lexicon of the 1910s as the equivalent of feminine patronymic ᴱQ. -wen (QL/103), but its only use in this period was in the masculine name ᴱQ. Indorildo, a variant of ᴱQ. Indorion and hence probably meaning “son of” (LT2/217). In The Etymologies of the 1930s Tolkien mentioned ᴹQ. -iel as a feminine patronymic under the root ᴹ√YEL “daughter” (Ety/YEL¹), but this root was rejected and in that document Tolkien seems to have replaced it with ᴹQ. -ien (EtyAC/YŌ).
In later writings Tolkien considered a bewildering variety of suffixes for the feminine patronymic, including -iel(d), -well, -wend and -ien (PE17/170, 190). In practice, though, only -iel appeared in actual names for “daughter of” (see above), perhaps because it is was the cleanest equivalent of the well-established masculine patronymic -ion “son of”.