willingly, through someone's will or volition (see Molina)
Alonso de Molina:
iyollocacopa. de su voluntad. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 36v. col. 2. Thanks to Joe Campbell for providing the transcription.