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.
Lockhart’s Nahuatl as Written:
also independent tēāchcāuh meaning not only someone's older brother but anything preeminent. also occurs as -tiāchcāuh and tāchcāuh. must have been pronounced tēyāchcāuh; the ē assimilated to the y, becoming ī, and the length of the i was neutralized; in this context the now short weak i led to yet another form, tāchcāuh. contains an incorporated tē- and -āch-, a root meaning first or elder; the remainder looks like the ending of a possessed pret. agentive noun. 232