achcauhtli.

Headword: 
achcauhtli.
Principal English Translation: 

older brother, warrior leader, ship captain, or person of authority (from Joe Campbell by way of H-Nahuatl listserv, 7/8/2022); leader of youths (see Karttunen); also, a constable (see Sahagún)

Orthographic Variants: 
āchcāuhtli, achcautli
IPAspelling: 
ɑːtʃkɑːwtɬi
Frances Karttunen: 

ĀCHCĀUH-TLI elder brother; leader of youths / primogénito, hermano mayor de alguien, el jefe de alguien (S) This seems to be related to ACHTO ´first,´but there is a vowel-length discrepancy. See ĀCH-TLI, TĀCHCĀUH, TIĀCHCĀUH.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 2.

Attestations from sources in English: 

in titecutli, in tachcauhtli in titequiva in ticalpixqui in titelpuchtli, auh in tipilli in titlapallivi yn titlamacazqui = you the lords, you the constables, you the valiant warriors, you the majordomos, you the youths, and you the noblemen, you the marriageable youths, you the priests (central Mexico, sixteenth century)
Fray Bernardino de Sahagún, Primeros Memoriales, ed. Thelma D. Sullivan, et al. (Norman: University of Oklahoma Press, 1997), 232.

Motenan, motzacuil. Inin tlatolli, intechpa mitoaya in tlatequipanoa: azo calpixcati, azo achcacauhti = Your wall, your enclosure. This was said of those who served in some capacity, such as tribute collectors or captains. Thelma D. Sullivan, "Nahuatl Proverbs, Conundrums, and Metaphors, Collected by Sahagún," Estudios de Cultura Náhuatl 4 (1963), 160–161.

achcautli = a military title
David Tavárez, The Invisible War: Indigenous Devotions, Discipline, and Dissent in Colonial Mexico (Stanford: Stanford University Press, 2011), 38.

And more from Joe Campbell by way of Ben Leeming, H-Nahuatl:
"Achcauhtli" can take specific possessive prefixes:
incihuatachcahuan = their head mistress of women
amotiachcauh = y'all's leader
imachcauh = their leader
m[o]achcauh = your older brother
***
One of the interesting facets in the "behavior" of
"te-achcauh" is the variation in the possessive prefix "te-".
1. It can be maintained in its full form:
teachcauh = captain
2. Its 'e' can raise to 'i':
intiachcahuan = their war leaders
3. Its vowel can completely elide:
intachcahuan = their war leaders

Attestations from sources in Spanish: 

achcauhtia Maxixcatzin = Maxixcatzin se constituyó como achcauhtli. (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), 120–121.

themes: 
Children, Childhood, Adolescence
Crime, Punishment
Family, Kinship, Descent
Marine, Maritime, Lacustrine
War, Conflict