Principal English Translation: 

in, at (locative suffix; tells where)

Frances Karttunen: 

-C(O) locative suffix; with stems ending in vowels this has the form –C among, in / dentro, en, sobre, por (S) –C(O) is a suffix which attaches to nouns. It does not take /K035/ the possessive prefixes as postpositions do. To indicate motion in the direction indicated by-C(O), -PA is suffixed to give the sequence -COPA.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 36.

Horacio Carochi / English: 

-co = general locative, relational word and suffix
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), 76-79, 500.

Lockhart’s Nahuatl as Written: 

general locative suffix. -c after a vowel. 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), 215.