the gray hair on an older woman's genitals (?) (see Molina)
Orthographic Variants:
ciuatl yacayo
Alonso de Molina:
ciuatl yacayo. el caño dela generacion dela natura dela muger. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 22v. col. 2. Thanks to Joe Campbell for providing the transcription.