the most delicate and soft feather of a bird (see Molina), perhaps referring to down feathers
Alonso de Molina:
tlachcayotl. la pluma mas delicada y blanda del aue. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 117r. col. 1. Thanks to Joe Campbell for providing the transcription.