in-between my thighs, the angle one's legs make when being born (see Molina)
Alonso de Molina:
maxac. no. entre mis piernas, en la horcajadura. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 54v. col. 2. Thanks to Joe Campbell for providing the transcription.