Bellaca is a slang term that translates to "horny" or "naughty." It's a word with a strong, explicit connotation, often used in reggaeton and trap music to describe someone who is sexually aroused or mischievous in a sexual way.
In the song, the lyric "En la cama bellaca, conozco tus mañas" (Naughty in bed, I know your tricks) directly uses this word to describe the partner's behavior in an intimate context. It's a very direct and somewhat provocative word that is central to the song's theme of a passionate, no-strings-attached relationship.
"Bellaka" is Xavi trading his usual heartbreak ballads for a flirty tug of war. He opens the song begging a girl to sneak out again, "Escápate esta noche otra vez" [Escape again tonight], and admits he actually prefers it when she isn't honest with him. The hook says it outright: "Miénteme, pero bonito" [Lie to me, but nicely]. He doesn't want the truth, he wants the show.
The rest of the song is him listing what he's willing to buy to keep that show going. She gets "Un cajón de Victoria's tuyos en la casa" [A drawer of your Victoria's Secret things at the house], rides around with "los vidrios oscuros pa' que no vean nada" [the tinted windows so nobody sees anything], and shows up in "Champañita rosé, bolsita Balenciaga-a" [rosé champagne, a little Balenciaga bag]. By the closing line, "En la cama bellaca, conozco tus maña-as" [In bed you're wild, I know your tricks], the title finally makes sense: bellaka is his nickname for her, part compliment, part warning.