Principal English Translation: 

above; over; on top of; at the top of; above the head of; often found on place names, but no necessarily, e.g. nocpac = above me
Joseph Augustin de Aldama y Guevara, Arte de la lengua mexicana (Mexico: Bibliotheca Mexicana, 1754), 25.

Orthographic Variants: 
-ticpac, -cpac.
Alonso de Molina: 

icpac. encima delo alto, o enlo alto de alguna cosa.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 33v. col. 2. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

-(I)CPAC postposition on or at the head of, /K094/ above / encima de lo alto, o en lo alto de alguna cosa (M), sobre, encima, en la cima, sobre la cabeza, delante (S)
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 94–95.

Horacio Carochi / English: 

-icpac = on top of
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), 502.

Lockhart’s Nahuatl as Written: 

-(i)cpac, relational word. on top of, above; on someone's head. see also -īxco.
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), 219.

Attestations from sources in English: 

The combining form can actually be reduced to -cpac, as in nocpac (atop me, on top of me, on my head). It can also combine with the ligature -ti-, making -ticpac, as in tepeticpac, atop the mountain, or tlalticpac, on the earth.
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 121.

tlalticpac = over the land; or, on earth;
oztoticpac = above the cave;
teicpac = over people
Note how these examples use the -ti- ligature.
Robert Haskett and Stephanie Wood's notes from Nahuatl sessions with James Lockhart and subsequent research.

Attestations from sources in Spanish: 

ycpac yn tepetl anotlalmacehual = en la cumbre del cerro, que es mi cacicazguía (Mimiahuapan, Tlaxcala, 1568)
Vidas y bienes olvidados: Testamentos indígenas novohispanos, vol. 2, Testamentos en náhuatl y castellano del siglo XVI, eds., Teresa Rojas Rabiela, Elsa Leticia Rea López, Constantino Medina Lima (Mexico: Consejo Nacional de Ciencias Tecnología, 1999), 148–149.

mocpac = encima de ti
nocpac = encima de mi
tepeticpac = encima del cerro
teticpac = encima de la piedra
Rebecca Horn's notes from Nahuatl classes with James Lockhart. Card file in possession of Stephanie Wood.