-c(a).

Headword: 
-c(a).
Principal English Translation: 

element, possibly itself a negative particle, used as a kind of ligature or punctuation between mā, tlā, and intlā and following negative particles and pronouns.

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), 212.