a day that is unhappy and without advantage; or, to not take full advantage of something or someone of intellectual or moral value (a saying) (see Molina)
IPAspelling:
neːsemilwitɬ
Alonso de Molina:
necemilhuitl. dia aziago y sin prouecho, o desaprouechado. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 64v. col. 1. Thanks to Joe Campbell for providing the transcription.