a light-colored egg; or, something that has already become putrid (see Molina)
Alonso de Molina:
opalan. hueuo guero, o cosa que ya esta podrida. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 77v. col. 1. Thanks to Joe Campbell for providing the transcription.