Aposentado literally means 'retired', usually referring to someone who has stopped working. However, in this song, Marisa Monte uses it in a beautiful and poetic way to describe the state of her heart.
She sings, "Já estava aposentado, sem nenhuma ilusão" (My heart was already retired, without any illusion). This metaphor suggests her heart had given up on love, much like a person retires from a career, making it a unique and memorable word in this context.
This is such a sweet and happy song! Marisa Monte sings about the incredible feeling of finding love when you've completely given up on it. She starts with a huge sigh of relief: "Ainda bem / Que agora encontrei você" (Thank goodness / I found you now). It's like she can't believe her luck, wondering what she did to deserve this person.
Before this new love, she felt totally alone and overlooked. She says "ninguém / Dava nada por mim" (nobody / bet anything on me), and she had even "desacreditei de mim" (lost faith in myself). Her heart was so used to being alone that she calls it "aposentado" (retired) and "maltratado" (mistreated). But then, this new person arrives and changes everything. They didn't just show up for a little while, "Você veio pra ficar" (You came to stay), making her happy and making her sing again.