Vacilão is a Brazilian Portuguese slang term that refers to someone who is a jerk, a fool, or someone who messes up frequently. It's a very common and expressive word in informal conversations and music.
In this song, the lyrics use vacilão to describe the ex-boyfriend who made a mistake by leaving the singer. It perfectly captures the feeling of exasperation and dismissal towards someone who acted foolishly in a relationship.
"Mostra Pra Ele" opens on betrayal: "Ele me prova que não vale nada" ["He proves to me he's worthless"], a partner who cheats and does it again. A friend cuts through the crying with a dare instead of sympathy — "Levanta a cabeça" ["Raise your head"], "Preocupa não, decepção não mata" ["Don't worry, disappointment doesn't kill"].
The chorus turns that dare into a plan: "Vamos sair pra balada" ["Let's go out to the club"], "Pegando e beijando na frente do ex" ["Making out and kissing in front of your ex"], daring him to watch — "Mostra pra ele a besteira que ele fez" ["Show him the stupid thing he did"]. By the end she's still needling him: "Quero ver se ele apronta outra vez" ["I want to see if he'll act up again"].