Principal English Translation: 

a connector that tells us that preceeding word is a verbal preterite agentive

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.

Horacio Carochi / English: 

-cā- = original preterit ending
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), 498.

Attestations from sources in English: 

When this ligature is used, the main verb will be on the right and the verb that is on the left is a modifier.