[Culo prieto]
Simply defined... a BLACK ASS. Used in Puerto Rico, to indicate, the color of the gluteus maximus in an individual, or, to describe, how dirty is the person's ass.
Oye, el novio de Giselle, se cree blanquito, pero mirale el culo prieto!
(Hey, Giselle's man thinks he's white, but, look at his black ass!)
(Hey, Giselle's man thinks he's white, but, look at his black ass!)