Afincado literally means "rooted" or "firmly planted," like a tree with deep roots. It's a very descriptive and unique word you won't hear in a song every day.
In "Loco Enamorado," it's used colloquially to describe a style of dancing. The lyric "se bailan lento, Bien pegado, afincado" (they are danced slowly, very close, rooted) paints a picture of two people dancing so closely and in sync that they seem planted to the spot. It’s a fantastic word that shows the poetic and expressive side of Spanish dance vocabulary.
This song is all about that moment you see someone across the room and just know you have to talk to them, even if you're shy. Abraham Mateo kicks things off by admitting he's not usually the one to make the first move, saying, "Te confieso que lo mío no es realmente hablar / Soy algo tímido como verás" (I confess that my thing isn't really talking / I'm a bit shy as you'll see). But for this person, he's willing to push past his nerves and confess everything.
The chorus is a super direct and catchy declaration of his feelings: "Ya me tienes como un loco enamorado / Baby, la verdad es que tú me gustas demasiado" (You already have me like a crazy man in love / Baby, the truth is I like you way too much). He's completely smitten and can't get her out of his head. Instead of just talking, he wants to show her how he feels by dancing close all night long, hoping she feels the same way he does.