a robust and round thing (toward a person or animal) or an animal's back (see Molina)
Alonso de Molina:
mimiliuhcayotl. redondez de cosa rolliza, o lomos de animal. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 56v. col. 2. Thanks to Joe Campbell for providing the transcription.