for a comet to fall in the sky; literally, for a year to fall (but see Molina)
Orthographic Variants:
xiuitl uetzi
Alonso de Molina:
xiuitl uetzi. (pret. xiuitl ouetz.) caer cometa. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 159v. col. 2. Thanks to Joe Campbell for providing the transcription.