A word for “lip” appearing only in its plural form pempi in 1964 notes on the parts of the mouth, where it was related to Q. pé “the closed mouth” (PE17/126). In 1968 notes on monosyllabic nouns, Tolkien said the primitive ✶pē “lip” was reduplicated to ✶pē̆pe; this is likely connected to an irregular plural form péti in the (untranslated) phrase et i péti “✱out of the mouth/lips”, with dissimilation of the second p to t as suggested by Patrick Wynne (VT47/35): ✱pēpi > péti. The 1964 form pempe is probably also the result of reduplication, likely ✱peñ-peñ > pempe(ñ), since √PEÑ was the usual root for “lip” (PE21/70; PE19/102).
Conceptual Development: The earliest “lip” word was ᴱQ. kilme from the Qenya Lexicon of the 1910s under the early root ᴱ√KILI “edge” (QL/46), a form also appearing in the contemporaneous Poetic and Mythological Words of Eldarissa (PME/46). A similar form ᴱQ. kilma “lip” appeared in a list of body parts from the 1920s, along with an alternate word ᴱQ. kaile “lip” (PE14/117). There are no signs of these early lip-words after that point.
Neo-Quenya: Tolkien also sometimes used Q. pé for “lip”; see that entry for discussion. For purposes of Neo-Quenya, I recommend using Q. pé mainly for “closed mouth” and pempë for a single “lip”. However, the dual form peu of pé was used of “the two lips, the mouth-opening” (VT39/9), which I would use for “a pair of lips for one person (open or closed)”, functioning as an irregular dual of pempë.
#pempë noun "lip" (attested only in pl. pempi, PE17:126); cf. pé.