Apodo translates to "nickname". It's an interesting word because it's not commonly featured in song titles or central themes, making it stand out.
In the song, the artist laments that his former lover gave their intense relationship a mere "nickname" like 'casi algo' (almost something) or 'una aventura' (an adventure), diminishing its significance. This use of apodo highlights the pain of a love that was profound for one person but casually dismissed by the other, giving it a deeper emotional weight than a simple nickname.
"Apodo" is Ryan Castro and Grupo Frontera splitting one complaint between them: an ex who's shrinking down what they had. The hook keeps circling back to "¿Por qué le pusiste apodo a este amor de nosotros?" [Why did you put a nickname on this love of ours?] because that's the whole argument in one line — she's calling it "casi algo", "una locura", "un polvito", "una aventura" [almost something, a bit of madness, a quickie, an adventure], four different labels for making it sound like less than it was.
He won't let the soft talk cancel out the rest either — "mi amorcito", "cariñito", "vida mía" [my darling, sweetie, my life] sat right next to "tantas veces nos comimos, es imposible que solo haya sido" [we slept together so many times, there's no way that's all it was]. When she leaves anyway, he doesn't beg, he just files it under "los amores se vienen y van, siempre vuelven como un boomerang" [loves come and go, they always come back like a boomerang] — a bet that the new nickname wears off before he does.