tzopelilia. nitla. (pret. onitlatzopelili.) endulcecer algo. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 153v. col. 2. Thanks to Joe Campbell for providing the transcription.
Frances Karttunen:
TZOPĒLILIĀ applic. TZOPĒLIĀ
TZOPĒLIĀ vt to sweeten something / lo endulza (T) This contrast with intransitive TZOPĒLIY(A) ‘to become sweet.’ Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 318.