a creature born standing up, or perhaps feet-first (see Molina)
Alonso de Molina:
moquetztiuh tlacat. criatura nacida de pies. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 060r. col. 1. Thanks to Joe Campbell for providing the transcription.