a first-born boy or girl (see Molina, who gives the example of the first person singular possessor, noyancuiyo, "my first born")
Alonso de Molina:
yancuiyo. no. mi primogenito o mi primogenita. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 30v. col. 2. Thanks to Joe Campbell for providing the transcription.