-tin.

Headword: 
-tin.
Principal English Translation: 

a plural ending added to nouns, when the noun has a consonant stem (see Lockhart)

Lockhart’s Nahuatl as Written: 

abs. pl. ending of nouns after consonant stems
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), 235.

Attestations from sources in English: 

Only animate beings (people and animals) were to be given the plural form in Classical Nahuatl. Typical plural endings are -tin and -meh (or, as Launey writes it, mê). Plurals can also be indicated through reduplication. Exceptions of inanimate plurals include mountains (tētepê) and stars (cīcitlāltin), which probably relate to their personification in religious belief. Also, -tin will follow a consonant, but the plural ending -meh (or, for linguists, -mê) will follow either a vowel or a consonant.
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 20–21.

pipiltin = children (pilli = child)

imeixtin Juramento quicelique = the three of them took oath (Tlaxcala, 1547)
Beyond the Codices, eds. Arthur J.O. Anderson, Frances Berdan, and James Lockhart (Los Angeles: UCLA Latin American Center, 1976), Doc. 22, 120–121.

Attestations from sources in Spanish: 

yn quenin oquitlalique otlanavatique cequintin Corregidoresme in imaltepepova ipan corregidorti = comó lo han dispuesto y ordenado otros corregidores en los pueblos en los cuales son corregidores (Cuauhtinchan, Puebla, s. XVI)
Luis Reyes García, "Ordenanzas para el gobierno de Cuauhtinchan, año de 1559," Estudios de Cultura Náhuatl 10 (1972), 264–265.

Cuando los sustantivos tienen las disinencias tzintli, tontli, etc., hacen siempre el plural en tin: ciuatzintli, mujer noble, ciuatzitzintin; piltontli, niñito, pipiltotontin; etc.
Rémi Siméon, Diccionario de la lengua náhuatl o mexicana (Mexico: Siglo XXI, 1988), xli.