Principal English Translation:
secondary particle; intensifies dubitative particles and interrogatives, often can be translated as the devil, in the world, in heaven's name, etc.
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:
mach (adverb) = an intensive particle
Daniel Garrison Brinton, Ancient Nahuatl Poetry: Containing the Nahuatl Text of XXVII Ancient Mexican Poems (1877), 157.