a ritual fasting or vigil; might also include bloodletting
Fasting is made visual with a special collar, such as the one that can be seen on the glyph for Nezahualcoyotl in Wikipedia, where there is a figure-8 shape below the coyote's chin.
Other glyphs for both Nezahualcoyotl and Nezahualpilli show possibly twisted fabric, in red, white, and green or red and green, described in a blog as "a colorful vertical band, topped by two or more vertical colorful bars." See the images reproduced on the blog.
The name Nezahual is found in early records from Cuernavaca, seen by Robert Haskett, and in the Matrícula de Huexotzinco, studied by Stephanie Wood.