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. Here are some examples of Nahua hieroglyphs of the name from the latter, which show a considerable range in visual representations of the name:
Nezahual (MH499v), the head of a man, https://aztecglyphs.wired-humanities.org/content/nezahual-mh499v
Nezahual (MH622r), the head of a man, https://aztecglyphs.wired-humanities.org/content/nezahual-mh622r
Nezahual (MH711r), a man in European clothing, walking, https://aztecglyphs.wired-humanities.org/content/nezahual-mh711r
Nezahual (MH633v), a hand clutching something, https://aztecglyphs.wired-humanities.org/content/nezahual-mh633v
Nezahual (MH664v), perhaps a tilmatli, https://aztecglyphs.wired-humanities.org/content/nezahual-mh664v
Nezahual (MH504v), a ceramic jug, https://aztecglyphs.wired-humanities.org/content/nezahual-mh504v
Nezahual (MH733r), seemingly sharp objects (forthcoming)