√PEÑ root. “*lip, mouth”
The word Q. pé was the main Quenya word for “lip(s)” for all of Tolkien’s life, but its derivation evolved over time. The root first appeared as unglossed ᴱ√PĒ in the Qenya Lexicon of the 1910s with the derivative ᴱQ. pé “the two lips, the (closed) mouth” (QL/72). The contemporaneous Gnomish Lexicon compared this Qenya word to G. beg “chin” or “beard” (GL/22), hinting that the actual root might have been *ᴱ√BĒ. In the Declension of Nouns from the early 1930s, the primitive form was given as PĒ “lips, mouth” (PE21/1, 38). In The Etymologies of the 1930s the root became ᴹ√PEG “(?outer) mouth” with derivative ᴹQ. pé “mouth” (Ety/PEG; EtyAC/PEG).
In Common Eldarin: Noun Structure from the early 1950s the root is given as unglossed √PEÑ (PE21/71), and in notes associated with the Quendi and Eldar essay of 1959-60 the primitive form is given as peñe (VT39/11). In both cases it was an example of a primitive form that resulted in ancient monosyllabic nouns from weak consonant loss: √PEÑ > ✶pē. In notes from the late 1960s Tolkien again gave ✶pē in a list of primitive monosyllabic nouns, but said “of these all except pe, su had probably lost a consonant in Common Eldarin”, implying the original form was actually √PĒ. But in green-note revisions made in 1970 to Outline of Phonology Tolkien had:
ñ disappeared prehistorically, so that words such as peñ were for Quenya long monosyllabic nouns with only an initial consonant: pē (PE19/102 and note #168).
Thus it seems the lost ñ in √PEÑ was restored, though it could also be a remnant of the earlier version of this sentence from the 1950s that gave both peñ and maʒ as examples of consonant-loss.
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is best to assume the root is √PEÑ.
References ✧ PE21/70; VT39/11
ᴹ√PEG root. “(?outer) mouth”
References ✧ Ety/PEG; EtyAC/PEG