Principal English Translation: 

merely, just, 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), 212.

Orthographic Variants: 
Frances Karttunen: 

ZĀ only / solamente (C) C distinguishes ZĀ from ZAN, which appears synonymous. According to C, ZĀ implies that the matter in question was not always as it is but has come to be so, whereas ZAN carries no such presupposition (Cf.110r). In T the reflex of ZĀ alternates in about equal numbers with ZA with a short vowel, which is the reflex of ZAN by T’s regular loss of word-final nasals. Z alternates between ZĀ and ZAH.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 345.

Horacio Carochi / English: 

ça = merely, just, etc.
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), 356–59, 498.