First time I’ve ever heard that, personally. I presume it has something to do with some idea that blacks aren’t humans. I’m guessing here, but it’s the only way I can fit the concept into a pejorative context.
I heard it on a Law & Order episode where a crooked cop used it when referring to the death of a hooker. From the context and discussion on the show, it seems it’s used to denigrate all sorts of people; druggies, people who aren’t white, the homeless, etc.
I’ve seen it used mostly in reference to crimes involving the homeless, hookers, drug addicts, etc.; basically, those who “respectable” people tend not to associate with.
Yes, that’s how I’ve heard the phrase - hookers, addicts, homeless. I never thought of it as being racist, but I’m in Canada where crime is probably perceived less as a racial thing and more as an economic issue.