Arrepio translates to "shiver" or "goosebumps." While it can refer to a shiver from cold or fear, in this song, it's used to describe the physical sensation of intense emotion, specifically love.
Paulinho Moska sings, "Corria em minha espinha um arrepio" (A shiver ran down my spine), conveying the overwhelming and almost physical impact of being deeply in love. It's a vivid and relatable word that captures the intensity of his feelings.
Paulinho Moska turns being in love into something like an illness here. He opens by calling it "uma coisa louca" [a crazy thing], the kind of state where you barely sleep and every minute apart from someone feels enormous. Then he pushes the idea further: "Tava tremendo, com febre, com frio" [I was trembling, with fever, with cold], as if love were a fever breaking his body down.
The hook says it plainly: "E eu nem pensava em mim, somente nela" [And I didn't even think of myself, only of her]. He laughs, he cries, he even calls the pain beautiful: "Nunca uma dor foi tão bela" [Never was a pain so beautiful]. By the end the whole song narrows down to two words, repeated over and over: "Somente nela" [Only of her]. Nothing else is left in his head.