Nuca means the back of the neck, or the nape. In the song, Liniker paints an intimate, memorable image of “a flor do lado da nuca” (“the flower beside the nape”), making this less-common body-part word feel poetic and personal.
Liniker sings this one from somewhere far away, missing someone so much that sleep has become the only way to see them: "Tenho dormido pra te encontrar" [I've been sleeping so I can meet you]. She is quick to say this trip was not a casual one, "Não é à toa, não sou turista" [It's not for nothing, I'm not a tourist], and that she wants to come back. What she wants to come back to is small and physical: "Ver seu cabelo crescer" [To see your hair grow] and "A flor do lado da nuca" [The flower by the back of your neck]. I love that detail. It is the kind of thing you only notice about someone you have looked at for a long time.
The title line is the heart of it: "E a melhor notícia foi viver você" [And the best news was living you]. "Viver você" is an unusual phrase in Portuguese too, treating the person as something you live through, like a season. That season has a date, "Naquele mês de agosto" [That month of August], when she "não dava nada no fim do rolê" [didn't expect a thing by the end of the night out]. Rolê is Brazilian slang for going out, a night on the town or just hanging around somewhere, and "não dar nada" means expecting nothing from it. So the big love story started on a night she thought would be forgettable.
There is some guilt in here as well, "Errar, errei, quem nunca?" [Mistakes, I made them, who hasn't?], and a plain admission that it might be over: "E se não tiver volta, foi demais viver" [And if there's no going back, living it was amazing]. "Foi demais" is everyday Brazilian for "it was awesome". She ends asking for one more chance to say "que eu tô viciado em você" [that I'm hooked on you], and on the final pass the song cuts off at "Poder dizer que eu tô" [To be able to say that I'm], leaving the sentence hanging. Cheeky, and it works.