nelli.

Headword: 
nelli.
Principal English Translation: 

a true thing, truly (adverb); the truth, the true one (noun)

IPAspelling: 
nelli
Alonso de Molina: 

nelli. cierto, ciertamente, o de verdad.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 66v. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

NEL-LI something true, certain / cierto, ciertamente o de verdad (M) In five attestations, B twice marks the E long, although other derivations from NEL-LI in B have the vowel specifically marked short. It is abundantly attested in C with no length marking at all. T has the reflex of a short vowel, while Z consistently has it long. This often occurs in the phrase HUEL NEL-LI 'something very true and certain, something genuine.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 164.

Horacio Carochi / English: 

nelli = a true thing, truly
Horacio Carochi, S.J., Grammar of the Mexican language with an explanation of its adverbs (1645), translated and edited with commentary by James Lockhart, UCLA Latin American Studies Volume 89 (Stanford: Stanford University Press, UCLA Latin American Center Publications, 2001), 507.

Andrés de Olmos: 

"Este tiene tres sentidos. Quando es aduerbio significa verdaderamente; quando es nomre substantiuo significa verdad; quando adjectiuo, cosa verdadera. Ex. : nelli niaz, cierto o verdaderamente yre; -- nelli otiquito, verdad dixiste; -- nelli tlatolli un onicac, palabra verdadera he oydo."
Andrés de Olmos, Arte para aprender la lengua Mexicana, ed. Rémi Siméon, facsimile edition ed. Miguel León-Portilla (Guadalajara: Edmundo Aviña Levy, 1972), pp. 184-5.

Attestations from sources in English: 

nelli teotl = true god -- Isabel Laack points out that this is a term coined by the missionaries to express a Christian monotheistic concept in Nahuatl.
Isabel Laack, Aztec Religion and Art of Writing (Leiden: Brill, 2019), 116.

y ça ce huellnelli dios = yet indeed one true God (San Bartolomé Atenco, 1617)
Beyond the Codices, eds. Arthur J.O. Anderson, Frances Berdan, and James Lockhart (Los Angeles: UCLA Latin American Center, 1976), Doc. 3, 58–59.

amo nelli ỹ ca ye tlapic = It is not true; it is made up (early seventeenth century, central New Spain)
Annals of His Time: Don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, James Lockhart, Susan Schroeder, and Doris Namala, eds. and transl. (Stanford: Stanford University Press, 2006), 66–67.

nelli mach = in truth, really all (an extreme intensifier)
Robert Haskett and Stephanie Wood's notes from Nahuatl sessions with James Lockhart and subsequent research.

Attestations from sources in Spanish: 

ei persona ça ce huel neli yn Dios ça centeotl = tres personas distintas y un solo Dios verdadero
Vidas y bienes olvidados: Testamentos indígenas novohispanos, vol. 1, Testamentos en castellano del siglo XVI y en náhuatl y castellano de Ocotelulco de los siglos XVI y XVII, eds. Teresa Rojas Rabiela, Elsa Leticia Rea López, y Constantino Medina Lima (Mexico: CIESAS, 1999), 218–219.

auh ynic yc nelli = Así, verdaderamente (Tetzcoco, 1587)
Benjamin Daniel Johnson, “Transcripción de los documentos Nahuas de Tezcoco en los Papeles de la Embajada Americana resguardados en el Archivo Histórico de la Biblioteca Nacional de Antropología e Historia de México”, en Documentos nahuas de Tezcoco, Vol. 1, ed. Javier Eduardo Ramírez López (Texcoco: Diócesis de Texcoco, 2018), 118–119.