ic.

Headword: 
ic.
Principal English Translation: 

with; when; for which reason
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.

IPAspelling: 
ik
Alonso de Molina: 

ic. con. preposicion.
ic. por esto, o por lo qual, portanto, o por esso. conjunction y aduerbio.
ic? en que tiempo? s. acaecio esso?
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 31v. col. 2. Thanks to Joe Campbell for providing the transcription.

Horacio Carochi / English: 

īc = when
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: 

interrogative. when, in terms of days, weeks, or longer periods of time.

ic. particle. in many ways acts like a 3rd person sing. relational word. for which reason, etc. creates ordinal numbers. gives an adverbial sense to adjectival substantives.

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: 

Àmo ōnicnāmic: ic ōninocuep = I didn't find him, so I returned
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 131.

Possibly a reduced form of ica (by it, thereby). Ic usually means "so, thus, under these circumstances" and "with."
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 131.

Ic also serves as a reference to a point in time: at what time, when did that happen? (see Molina)

Īc has a longer form with the same meaning, īquin, when.
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 133.

yvan yc = And at this time
James Lockhart, We People Here: Nahuatl Accounts of the Conquest of Mexico, Repertorium Columbianum v. 1 (Los Angeles: UCLA Center for Medieval and Renaissance Studies, 1993), 282–283.