useless, idle, or lazy woman, born during the five unlucky/useless days at the end of the year that were associated with darkness and feminine spiritual forces. This is explained in the Florentine Codex in Book 2, folio 12 recto (see: https://florentinecodex.getty.edu/book/2/folio/12r). And see the attestations, below.
Useless woman, unhappy because she was born during one of the five complementary days called "nêmontemi." The male equivalent is "nêmoquichtli." (Translated by Stephanie Wood from the French by A. Wimmer (2004): "Femme inutile, malheureuse parce qu'elle est née durant l'un des cinq jours complémentaires appelés 'nêmontemi'. correspondant masculin, 'nêmoquichtli'."
According to Heriberto Martínez Yépez, the five-day period was seen as a time when the "sun suspended its normal activity" ("suspenderse el régimen del sol"), and this could happen at the end of the calendar or during an eclipse ("por el fin del calendario o por un eclipse"). This is when there was darkness ("oscuridad") and feminine entities became active ("entidades femeninas se activan"). This activity was associated with the West, which in Nahuatl is called Cihuatlampa ("Rumbo de las mujeres"). He goes on to suggest that the subsequent use of the "nen" syllable to mean "in vain" or "uselessly" comes from a "patriarchal definition of femeninamente" ("una definición patriarcal de 'femeninamente'"). Finally, he says, these are "feminine days, woman-days, days of feminine forces, goddess-days" ("días femeninos, días-mujer, días de fuerzas femeninas, días-diosas").