Desandou is the past tense of the verb desandar, which literally means "to undo what has been walked" or "to retrace one's steps". However, in common Brazilian Portuguese, it's frequently used to mean that something "went wrong," "fell apart," or "deteriorated."
In the context of Djavan's song, after a beautiful beginning, the line "Você foi prum lado e eu pro outro / Aí desandou" perfectly captures the moment when the relationship took a turn for the worse and everything fell apart. It's a concise and evocative word for a relationship's demise.
Have you ever had a relationship that just... fell apart, and you're left wondering what happened? That's exactly what Djavan is getting at in "Desandou." He's looking back at a love that started so beautifully, remembering how they were so enchanted with each other. He even describes his partner as having "a lã do primeiro amor" [the wool of first love] in their eyes, which is such a sweet image of innocence and newness.
He talks about feeling like he had everything he wanted, those "dias imaculados" [immaculate days]. He tried to steal a kiss, but what he got was something "indescritível" [indescribable] – it completely swept him away! But here's the twist: he says, "nesse beijo só tinha alegria, faltava dor" [in that kiss there was only joy, pain was missing]. It's almost like it was too perfect, or maybe it lacked a certain depth. And then, just like that, "Você foi prum lado e eu pro outro, aí desandou" [You went one way and I went the other, then it went wrong]. It's such a blunt and relatable way to describe a relationship suddenly unraveling. He's still asking, "Me diga já, o que foi que aconteceu?" [Tell me now, what happened?], trying to make sense of how something so good could just... desandar.