Principal English Translation:
not, un-, in- (often used as a negative prefix; see Karttunen)
Frances Karttunen:
AH- negative prefix not, un-, in- / negación, usado en comp. por amo, no (Siméon)
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 3.
Horacio Carochi / English:
à- = (negative subparticle)
temporal particle inserted after in particle clusters, 360–61
sometimes loses glottal stop, 359 n5, 454 n4
never stands alone, 405 n5
as an interrogative, 446–47
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), 108–09.
Lockhart’s Nahuatl as Written:
not
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), 210.
Attestations from sources in English:
à (or ah) is a particle that comes at the beginning of some words and regularly has a negative sense (as in àmo), but which originally intended to insert doubt (as in àzo), says Michel Launey. This particle is not always used as a prefix.
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 129.
Àzo quichīhuaz, ànozo àmo = Maybe he'll do it, maybe not
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 129.