Bellaco is a slang term that can mean 'horny' or 'mischievous', often with a connotation of being sexually aroused or having a playful, cunning nature.
In this song, the artist uses it to express being intensely aroused and driven by desire, specifically when he sings, "Me tienes aquí bellaco, escribiéndote" (You have me here horny, writing to you). It's a common word in reggaeton and urban music to convey strong sexual longing.
Have you ever been out, seen someone you used to know, and instantly all those old feelings come rushing back? That's exactly what's happening in Brytiago's "Por El Alcohol." Our singer spots an old flame and can't help but wonder, "Hola, ¿cómo está'? Por lo que veo tú te ve' muy bien" [Hey, how are you? From what I see, you look very good]. He's curious if she's single or with someone new, because he's ready to escape with her again, just like old times.
He's been looking for her, thinking about her every day, and now that he's found her, he wants to relive their past. He admits, "Y es que veo tu foto y me debilita por el alcohol" [And it's just that I see your photo and it weakens me because of the alcohol], suggesting that seeing her picture, especially after a few drinks, makes him want her even more. He's clearly obsessed, describing her physical features in detail and confessing, "Te juro que yo me volví adicto a tus partes" [I swear I became addicted to your parts]. He's stuck in a loop, missing her and even touching himself while thinking about her, hoping she'll answer his calls and messages for one more night together. It's a pretty direct and intense look at longing and desire!