cuitlapilli.

Headword: 
cuitlapilli.
Principal English Translation: 

a tail, such as that of an animal or a bird (see Molina); also the tail of a comet (see Zapata y Mendoza); and, part of the metaphor for commoner(s), cuitlapilli atlapalli (see Lockhart)

IPAspelling: 
kwitɬɑpilli
Alonso de Molina: 

cuitlapilli. cola, o rabo de animal, o de aue.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 27r. col. 2. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

CUITLAPIL-LI tail / cola o rabo de animal o de ave (M) This is frequently attested in B with the vowel of the last syllable specifically marked short, but also twice with the vowel marked long, both times before the plural suffix -HUĀN. It is marked short in T but long in Z. See the discussion under PIL-LI.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 73.

Lockhart’s Nahuatl as Written: 

part of the expression in cuitlapilli in ahtlapalli, a metaphor for the common people. cuitlatl, excrement or extrusion, pilli.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 216.

Attestations from sources in English: 

Injc Vme Parrapho: ipan mjtoa, in quenjn qujnpepenaia Iuezes. In tlatoanj oc cenca qujmocujtlaujaia in tetlatzontequjliliztli, qujcaquja in jxqujch in jneteilhujl: ioan in jchoqujz, in jnentlamachiliz in jnetolinjliz in cujtlapilli, atlapalli in jcnotlacatl, in motolinja in maçeoalli = Second Paragraph, in which it is described how they choose judges. The ruler watched especially over the trials; he heard all the accusations and the complaints, the afflictions, and the misery of the common folk, the orphans, the poor, and the vassals (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 8 -- Kings and Lords, no. 14, Part IX, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1951), 54.

Cuitlapilli, in atlapalli. Quitoznequi: maceoalli = The tail and the wing. This means the common people.
Thelma D. Sullivan, "Nahuatl Proverbs, Conundrums, and Metaphors, Collected by Sahagún," Estudios de Cultura Náhuatl 4 (1963), 146–147.

ipan qujҫoҫoaco in jahaz, in jcujtlapil, in onelli mach ipan mohonoltitivia = he came spreading his wings, his tail feathers over it; truly he spread himself over it (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 23.

tiqujmjle, ticacaxe tiez, tehoatl timalacaioaz, tehoatl tecauhiooaz, ticeoalloaz: motlan mocalaqujz in cujtlapilli, in atlapalli = thou art to be the one with the bundle, the carrying frame. Thou art to be the umbrage, thou art to be the shade, the shadow, beneath which the vassals are to enter (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 184.

matheo çivācuitlapil (the glyph next to the gloss of the name shows a woman's head, cihuatl, and a tail, cuitlapilli) (Tepetlaoztoc, sixteenth century)
Barbara J. Williams and H. R. Harvey, The Códice de Santa María Asunción: Facsimile and Commentary: Households and Lands in Sixteenth-Century Tepetlaoztoc (Salt Lake City: University of Utah Press, 1997), 100–101.

Attestations from sources in Spanish: 

cuitlapilli, atlapalli = los vasallos, los macehuales (sixteenth century, Quauhtinchan)
Historia Tolteca-Chichimeca, eds. Paul Kirchhoff, Lina Odena Güemes, y Luis Reyes García (México: CISINAH, INAH-SEP, 1976), 153, nota 5.

opeuhqui çitlali popoca çanoyapa quitecaya yn iuhqui icuitlapil = empezó a humear la estrella, por todas partes del cielo aparecía lo que parecía su cola (Tlaxcala, 1662–1692)
Juan Buenaventura Zapata y Mendoza, Historia cronológica de la Noble Ciudad de Tlaxcala, transcripción paleográfica, traducción, presentación y notas por Luis Reyes García y Andrea Martínez Baracs (Tlaxcala and México: Universidad Autónoma de Tlaxcala, Secretaría de Extensión Universitaria y Difusión Cultural, y Centro de Investigaciones y Estudios Superiores en Antropología Social, 1995), 344-345.

in que[n] quihuicazq[ue] yn icuitlapil yn iatlapal = cómo conducirán a su pueblo [la expresión viene del huehuetlatolli, el discurso antiguo] (ca. 1582, México)
Luis Reyes García, ¿Como te confundes? ¿Acaso no somos conquistados? Anales de Juan Bautista (Mexico: Centro de Investigaciones y Estudios Superiores en Antropología Social, Biblioteca Lorenzo Boturini Insigne y Nacional Basílica de Guadalupe, 2001), 198–199.