tzintli.

Headword: 
tzintli.
Principal English Translation: 

base, foundation, starting point; buttocks, bottom, anus (see Karttunen); pictured visually in glyphs as the lower half of the body, in place names tzintli can also refer to a lower place, such as at the foot of a hill, or a smaller place, given that -tzin can be a diminutive and not just a reverential; -tzinco can be a suffix used for spin-off towns that re-use an original place name (Karttunen)

IPAspelling: 
tsiːntɬi
Alonso de Molina: 

tzintli. el ojo del saluonor.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 152v. col. 2. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

TZĪN-TLI base, foundation; buttocks, anus / el ojo del salvohonor (M), ano, colon, cimiento, base (S) In the sources for this dictionary this appears only in compounds, but it is given as a free form in M.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 314.

Lockhart’s Nahuatl as Written: 

bottom, base, anus
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), 240.

Attestations from sources in English: 

tzintzonqualactli = rectal humor
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 10 -- The People, No. 14, Part 11, 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), 138.

ytzitla copalquauhuitl = under the copal tree (San Bartolomé Tlatelolco, Toluca Valley, 1715)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 92.

tepetl ytzintla = at the foot of the hill
R. H. Barlow, The Techialoyan Codices; Codex J (Codex of Santa Cecelia Actitlan), Tlalocan 1:3 (1944), 232.

tlatzintla = below

tzintli = buttocks
totzin = our buttocks
omio = bony
nacaio = fleshy
nacaiooa = they become fleshy
omiiooa = they become bony (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 10 -- The People, No. 14, Part 11, 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), 122.

amo tzintopoltic, amo tzincuecuexactic, tzintamalcuecuexac = not of hatchet-shaped buttocks; nor of flabby buttocks or thighs (16th century, Mexico City)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 2—The Ceremonies, No. 14, Part III, 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), 65.

-tzitzin = plural
-tzitzintin = plural for animates
Rebecca Horn's notes from Nahuatl classes with James Lockhart, being harvested for this dictionary by Stephanie Wood.

Attestations from sources in Spanish: 

oca peuhqui yn itzintlan Oçelutepetl opan ticahuato Xoxohuiquitepec ynic motocac cohuatepatl = El recorrido del lindero común empezó ahí al pie del Ocelotepetl, y fue a terminar en Xoxouhquitepetl (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), 352–353.

Este siempre se entrepone al verbo y quiere dezir: atras o hazia atras. Ex. nitzinquiça, torno atras, o salgo hazia atras.
Paso y Troncoso, 1904, 102.