Literal translation
to be a skewer
Meaning
to be a worthless, untrustworthy person
Explanation
A broco originally means a small shoot sprouting from the base of a plant, an unwanted growth that gardeners call a sucker because it drains the plant's sap. People say marrido broco, bad shoot, of a child gone wrong, and the negative sense eventually stood on its own. Calling someone uno broco compares them to that shoot needing to be cut away: a worthless person, untrustworthy, good for nothing. It is a final, fatalistic insult, harsher than a mere good-for-nothing, who can at least remain likeable.