“Malo” is the adjective (describing word) which normally means “bad” but can also mean “ill”, “poor”, “wrong”. It also has a feminine form, “mala” as well as plural forms, “malos” and “malas”.
Look at these examples:
Empecé a leer un libro tan malo que no podía terminarlo.
I started to read such a bad book that I couldn’t finish it.
¡Mala suerte!: Bad luck! [suerte is a feminine noun] Continue reading