teachcauh.

Headword: 
teachcauh.
Principal English Translation: 

older brother or cousin (see also tiachcauh)
Beyond the Codices. eds. Arthur J. O. Anderson, Frances Berdan, and James Lockhart. Berkeley, O.C. Press: 1976, p24.

Orthographic Variants: 
tiachcauh
Alonso de Molina: 

teachcauh. hermano mayor, o cosa mayor mas excelente y auentajada.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 91r. col. 2. Thanks to Joe Campbell for providing the transcription.

Lockhart’s Nahuatl as Written: 

nachtzin = the older brother of a woman specifically
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), 48.
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), 232.

Attestations from sources in English: 

teachcahuan = leaders or again elders
Here in This Year: Seventeenth-Century Nahuatl Annals of the Tlaxcala-Puebla Valley, ed. and transl. Camilla Townsend, with an essay by James Lockhart (Stanford: Stanford University Press, 2010), 60.

older brothers; masters of youth -- (pl.) teachcahuan
Susan Kellogg, Law and the Transformation of Aztec Culture, 1500–1700 (Norman and London: University of Oklahoma Press, 1995), 226.

Auh ca iz tica in titeach in tiacapãtli, auh ca iz tõca in titlacoieoa, auh in titlatoqujlia; auh iz tica ompa tica on in tixocoiutl = And here standest thou who art the oldest, the firstborn; and here art thou who art the second; and thou who followest; and thou who standest, who standest there, thou who art the youngest (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), 87.

Attestations from sources in Spanish: 

yn oncan escuela yn iquac nintiyachcauh nicatca = que cuando era yo mayor en la escuela (Coyoacan, 1607)
Vidas y bienes olvidados: Testamentos en náhuatl y castellano del siglo XVII, vol. 3, Teresa Rojas Rabiela, et al, eds. (México: CIESAS, 2002), 56–57.

ynoteachcauh ytoca Fel[i]ciano = mi hermano menor que se llama Felisiano (Ocotelulco, 1593)
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), 234-235.

yn iachcauh = su hermano mayor (Tlatelolco, 1558)
Luis Reyes García, Eustaquio Celestino Solís, Armando Valencia Ríos, et al, Documentos nauas de la Ciudad de México del siglo XVI (México: Centro de Investigación y Estudios Superiores en Antropología Social y Archivo General de la Nación, 1996), 80.

themes: 
Family, Kinship, Descent