nahuallatolli.

"The Nahua called that intricate form of communicating with invisible forces nahuallatolli which means ‘secret word,’ because it derives from nahualli (the hidden, the covered) and tlatolli (speech, word).”
Alfredo López Austin, The Rabbit on the Face of the Moon: Mythology in the Mesoamerican Tradition, translated by Bernard R. Ortiz de Montellano and Thelma Ortiz de Montellano. Salt Lake City, University of Utah Press. 1996, p. 27. Shared by James Maffie.

nahuallatolli = “language of the occult”
Alfredo López Austin, Tamoanchan, Tlalocan: Places of Mist, translated by Bernard R. Ortiz de Montellano and Thelma Ortiz de Montellano. Niwot: University Press of Colorado, 1997, p. 30. Shared by James Maffie.

"Nahuallatolli was the 'language of the sorcerers' (Jansen 1985:6) and the 'principal credential for validating a person’s entry into the powerful, ethereal realm' (López Austin 1967:1) as he transforms himself into a tlamacazqui. In the context of incantations, the word tlamacazqui alludes to all the recipients of these chants (for example, water and the goddess of water), but at the same time it also refers to the sorcerer himself. Jacinto de la Serna (1953/1892) translates this word in the seventeenth century into Spanish as espiritado ('possessed'), a word that today has more the meaning of 'charmed,' 'bewitched,' or 'possessed by the divine spirit.'
Katarzina Mikulska Dubrowska, "'Secret Language' in Oral and Graphic Form: Religious-Magic Discourse in Aztec Speeches and Manuscripts," Oral Tradition, 25/2 (2010): 325-363. Available from: https://www.researchgate.net/publication/236786259_Secret_Language_in_Or... [accessed Jul 18 2020].

This type of speech was the "principal credential for validating a person's entry into the powerful, ethereal realm."
Alfredo López Austin, "Términos del nahuallatolli," Historia mexicana 17 (1967), 1–36; see page 1.