Pirul refers to the Peruvian pepper tree, a common tree in Mexico. It's a specific and somewhat uncommon word in song lyrics, making it memorable.
In the song, the phrase "Leña de pirul" (pepper tree wood) is used metaphorically to describe someone useless or worthless, as the wood of the pirul tree is known for not burning well. It's a vivid and culturally specific insult, adding a unique flavor to the lyrics.
Have you ever had that one ex who just can't seem to get it together after you break up? That's exactly what La Santa Cecilia is singing about in "Leña De Pirul." The song kicks off with the singer complaining that whenever people talk about their ex, it's always to make them cry, to give them a complaint, to say that things have gone very badly for them. The singer even warned them, saying, "Solo te van a engañar" [They are only going to deceive you]. Now, the ex is going from failure to failure, mentioning the singer's name, which, of course, makes the singer cry.
But then, the song takes a turn from sadness to a bit of exasperation and a desire for peace. The singer starts asking, "Cuanto quieres por que me olvides / Cuanto quieres por que te alejes" [How much do you want to forget me / How much do you want to go away]. It's like, just name your price to get out of my life! The most striking part is when the singer calls the ex "Pobre Leña de pirul / Que no sirves ni pa' arder" [Poor pirul wood / That isn't even good for burning]. That's a pretty blunt way to say someone is useless, only good for making the singer cry. It's a mix of frustration, a little bit of lingering hurt, and a strong desire to just move on.