Terrível literally means "terrible" or "awful." However, in Brazilian Portuguese slang, especially in a party or social context, it can take on a very different, almost opposite meaning.
In this song, when Cristiano Araújo sings "Hoje eu tô terrível," he means he's feeling incredibly wild, mischievous, and ready to cause some trouble or have a crazy good time. It implies he's in a mood to break rules, flirt, and generally be a bit of a rascal, rather than feeling bad or unwell. It's a fun example of how context completely changes a word's meaning!
A woman turns Cristiano Araújo down, and he spends the song shrugging it off in front of everyone. "Ela não quis me namorar, perdeu" ("She didn't want to date me, she lost out") sets the tone: her loss, not his. He's "muito bem solteiro" ("very well single"), women are "cheio... a minha volta" ("all around" him), and — the line that turns the rejection into a dare — he's already gone and gotten with her friend: "Eu já peguei sua amiga, mas quero você também" ("I already hooked up with your friend, but I want you too").
The title line is the joke the whole song is built on. "Terrível" usually means "terrible," but here it's sertanejo slang for feeling unstoppable — "hoje não vai prestar" ("today it's going to be wild"). He's off the market for anything serious ("Pra romance é off," "for romance, I'm off") but fully available for the party ("pra balada é disponível," "for partying, I'm available"). It's less heartbreak than performance: a breakup turned into a brag, sung at full volume for a room that's clearly on his side.