Principal English Translation: 

on a side

Frances Karttunen: 

-TLAPAL side / lado (K) When this compounds with a quantifier to form a directional construction, as in ŌNTLAPAL ‘from both sides,’ it does not take an absolutive suffix, but it is apparently the second element of AHTLAPAL-LI ‘leaf, wing’ which does take one.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 288.

Horacio Carochi / English: 

-tlapal = on a side
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), 340–43, 514.