Principal English Translation: 

reverential ending used with verbs already in the reflexive

Frances Karttunen: 

-TZINOĀ compounding verbal element used to form the honorific of reflexive verbs. The same vowel length problems across sources discussed under –TZIN-TLI apply to –TZINOĀ, since the latter is a derivation of the former. See the discussion under –TZIN-TLI.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 313.

Lockhart’s Nahuatl as Written: 

Class 3 reverential suffix of verbs, used sometimes over and above the normal reverential, but especially when the reflexive prefix has already been used in a semantically meaningful fashion. pret. -tzīnoh.
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), 240.