Bellaco is a Spanish slang term that can mean 'horny' or 'mischievous', often with a connotation of being cunning or sly.
In the context of this song, Justin Quiles uses it to describe the effect a woman's tattoo has on him, making him feel 'bellaco'. It's a word that captures a raw, intense feeling and is quite common in reggaeton and urban music, offering a glimpse into a more colloquial and expressive side of the Spanish language.
Justin Quiles spends "Su Gato" on one boast he keeps returning to: "Yo se la robé a su gato" ("I stole her from her man"). Gato is Puerto Rican street slang for the boyfriend she came with, and the line lands as a punchline rather than a confession. The setup arrives as a question from off-mic — "Mera, Quile', ¿Pero y esa baby?" ("Hey, Quile', so what's the deal with that girl?") — and the rest of the track is his answer.
What complicates the swagger is how watchful the verses are. He takes her "por la cintura" ("by the waist") while keeping an eye on the door, "por si el novio se aparece en lo oscuro" ("in case the boyfriend shows up in the dark"), then tells her she is safe because she's "con los duro'" ("with the tough guys"). The beat stays glossy and unhurried; the words keep checking over their shoulder.