This phrase literally means “distributing pieces of a raw pig” and refers to when someone is framed for something he or she did not do. The phrase has its origins in the pig cutting traditions commonly found in rural areas during Chinese New Year and other holidays. When Chinese villages celebrate the new year, the elders of the village slaughter a pig and hand out pieces of the pig to all the villagers. The meat is handed out randomly, and whichever piece you get is simply the luck of the draw.
Back during a time of rampant police corruption in Hong Kong, this phrase was used to describe a situation when the police would place someone’s case file on the table and fabricate whatever evidence they needed to arrest their target. Whatever charges would get thrown at their target was simply the luck of the draw. Those who met such a misfortune would often complain 係咪砌生猪肉?!