This is the 711th most frequent Spanish word.
peor
"Worse."
Here, 'peor' is being used as a superlative adjective to describe the day as the worst in comparison to others.
Este ha sido el peor día de mi vida.
This has been the worst day of my life.
In this case, 'peor' functions as a comparative adjective denoting a decline in condition or state.
Las cosas pueden ponerse peor si no actuamos rápido.
Things can get worse if we don't act quickly.
Here, 'peor' is used as a noun, referring to the worst possible outcome.