Ajena translates to "belonging to another" or "someone else's." It's a poignant word that captures the central conflict of the song.
The singer is deeply attracted to someone, but she is already taken, making her "ajena." This word perfectly encapsulates the bittersweet longing and the forbidden nature of his desires, as he laments, "Lástima que seas ajena" (It's a shame that you belong to someone else).
This song is all about a man who is completely smitten with someone, but there's just one big problem: she's already taken! He can't help but confess, "Me gustas completita" (I like all of you). Just saying hello gives him "el mal del amor" (the sickness of love), making his body tremble and his desires overflow. He's imagining all sorts of beautiful things with her, so much so that he finds other people around them annoying because they're getting in the way of his daydreams!
He keeps repeating, "Lástima que seas ajena" (It's a shame you belong to someone else), because he can't give her "lo mejor que tengo" (the best I have). He feels like he arrived too late and doesn't have "llave para abrir tu cuerpo" (a key to open your body). He even calls her "el fruto prohibido que jamás comí" (the forbidden fruit I never ate). He's so into her that he says for someone like her, he'd "dejo todo" (leave everything), promising to take her "al mismo cielo" (to heaven itself) if only she weren't with someone else. It's a classic tale of unrequited love, full of longing and a bit of playful frustration!