This duet between Naiara Azevedo and Kadu Martins is angry, blunt and a little bit rude, in a fun way. The ex has come crawling back, and the answer is a hard no: "Agora quer o coração que tu deixou na merda" (Now you want the heart you left in the dirt, literally "in the shit"). Then the punchline: "Nem se eu tivesse outro reserva" (Not even if I had a spare one). Reserva here means a backup, like a spare tyre.
The chorus keeps asking "Por que tu não me amou? Por que tu só me usou?" (Why didn't you love me? Why did you only use me?). And the kiss-off line is "Enfia essa saudade onde tu merece" (Shove that longing where you deserve), basically telling them where to stick it. "Já teve sua chance, agora me esquece" (You already had your chance, now forget me). Notice the tu with third-person verbs like tu foi and tu queria: that's very common informal Brazilian speech, even if grammar books frown at it.