to cover something with a shadow or shade, or to make a shade or shadow (see Molina)
Alonso de Molina:
yacalhuia. nitla. (pret. onitlayacalhui.) cubrir algo con sombra, o hazer sombra. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 30r. col. 1. Thanks to Joe Campbell for providing the transcription.