machtli.

Headword: 
machtli.
Principal English Translation: 

nephew, niece (of a man)

IPAspelling: 
mɑtʃtɬi
Alonso de Molina: 

machtli. sobrino, de varon.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 50v. col. 2. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

MACH-TLI nephew, or niece (from the point of view of an uncle) / sobrino, de varón (M) [(7)Bf.4V,5R,12R,12V]. S glosses this as specifically ‘nephew,’ but according to the Spanish-to-Nahuatl side of M, it applies equally to nephews and nieces.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 128.

Attestations from sources in English: 

ym machtli tlaua, auiua ycnotl, tlacnocaualli, tetloc tenauac nemini, tetlan nenenqui tetlan nenqui = A nephew (niece) has an uncle [or] an aunt. [He is] an orphan - parentless - who serves in another's house, a servant; one who lives with others. (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), 4.

oualla imach julia vixtopolcatl = came the niece of Julián Uixtopolcatl (Coyoacan, 1554)
Beyond the Codices, eds. Arthur J.O. Anderson, Frances Berdan, and James Lockhart (Los Angeles: UCLA Latin American Center, 1976), Doc. 10, 90–91.

yz cate ymachva teveltecac ometi = Here are Tehueltecac's two nieces/nephews. (Cuernavaca region, ca. 1540s)
The Book of Tributes: Early Sixteenth-Century Nahuatl Censuses from Morelos, ed. and transl. S. L. Cline, (Los Angeles: UCLA Latin American Center Publications, 1993), 160–161.

Ōtinēchmocnēlli nomăchtze = Thank you, my nephew. (Central Mexico, 1570–80)
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), 197.