maca.

Principal English Translation: 

let no, let not; may not (something happen); be not; don't; no

Orthographic Variants: 
macamo, ma ca
IPAspelling: 
mɑːkɑ
Alonso de Molina: 

maca. no. s. de imperatiuo, o de auisatiuo vetatiuo. y es macamo sincopado.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 50r. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

MAHCA negated form of MAH such that not, as though it weren’t / y que no (C), no (M) This has a counterfactual implication ‘not as things in fact are.’ Double negations with MAHCA affirm; the sense of AHMŌ MAHCA is ‘without doubt’ (Cf.123V). This contrast with MĀCA, the negative counterpart of the clause-introductory particle MĀ . See MAH.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 130.

Horacio Carochi / English: 

māca = negative particle with optative
màca = that not
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), 505.

Lockhart’s Nahuatl as Written: 

māca. particle. with çan, not just. also in other negative expressions. mā, ca as in mācamō etc. 223
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), 223.

Attestations from sources in English: 

maca xiquinnotzacan. = Do not speak to them. (central Mexico, early seventeenth century)
Codex Chimalpahin: Society and Politics in Mexico Tenochtitlan, Tlatelolco, Culhuacan, and Other Nahuatl Altepetl in Central Mexico; The Nahuatl and Spanish Annals and Accounts Collected and Recorded by don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, eds. and transl. Arthur J. O. Anderson and Susan Schroeder (Norman: University of Oklahoma Press, 1997), vol. 1, 92–93.

xiccujtivetzi in ochpanoaztli in ochpanalli, itlan xaquj: maca xiiamanjxto, maca xitotojxto = Seize the broom: be diligent with the sweeping; be not tepid, be not lukewarm (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 95.

momaca = give oneself to (central Mexico, sixteenth century)
Personal communication, James Lockhart, in sessions analyzing Huehuetlatolli.