ach.

Headword: 
ach.
Principal English Translation: 

dubitative particle indicating possibility, not knowing (see Karttunen); a particle indicating ignorance with interrogative words; emphasizing casual expressions; exclamation of surprise (see Carochi)

IPAspelling: 
ɑtʃ
Frances Karttunen: 

ACH dubitative particle possibly, one doesn´t know / part. que ordinariamente indica duda y a veces equivale a una negación (S) Cf.125v gives ACHAH as a variant. Zp.139 gives ACHA ~ ĀCHĀ as variants.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 2.

Horacio Carochi / English: 

ach = (particle indicating ignorance with interrogative words; emphasizing casual expressions; exclamation of surprise)
Horacio Carochi, S.J., Grammar of the Mexican language with an explanation of its adverbs (1645), translated and edited with commentary by James Lockhart, UCLA Latin American Studies Volume 89 (Stanford: Stanford University Press, UCLA Latin American Center Publications, 2001), 334–35, 370–73, 406–07, 408–11, and 496.

Attestations from sources in English: 

ach (dubitative particle) = indeed? is it not? etc.
Daniel Garrison Brinton, Ancient Nahuatl Poetry: Containing the Nahuatl Text of XXVII Ancient Mexican Poems (1887), 149.

ach ça ye nelli = exclamation of surprise (the a of ça should have a macron)
Horacio Carochi, S.J., Grammar of the Mexican language with an explanation of its adverbs (1645), translated and edited with commentary by James Lockhart, UCLA Latin American Studies Volume 89 (Stanford: Stanford University Press, UCLA Latin American Center Publications, 2001), 408–11 and 496.

Ach zan ninomati = Possibly, I only know myself
John F. Schwaller, "The Pre-Hispanic Poetics of Sahagún's Psalmodia christiana," in Psalms in the Early Modern World, eds. Linda Phyllis Austern, Kari Boyd McBride, and David L. Orvis (London: Ashgate, 2011), 325.