a really crazy, foolish thing; an interjection (see Molina)
Alonso de Molina:
iuian. desatino y locura grande. interjection. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 44r. col. 1. Thanks to Joe Campbell for providing the transcription.