Jacques Lacan states that the unconscious is structured like a language. Language is central to the ways in which we understand the world: our grammar, our vocabulary, our syntax. Language is also descriptive and formative: if we describe an object as slimy, eventually that descriptor forms a powerful association in our culture (e.g. lawyers). It doesn't take a genius to understand that centuries of racist thought formed powerful associations in the way the dominant culture of the United States looked upon minorities, in particular Black minorities. What descriptors attached themselves to Blacks through language: lazy, smelly, unintelligent, child-like...the U.S. had a fine tradition of the "minstrel show" that operated on these overdetermined qualities.
Of course, Blacks aren't alone in having been hated minorities. They just are lucky enough to be long-standing and continuous targets. In their time, the Irish were also seen as sub-human, and in the early 20th century, one of the greatest threats apparently facing our nation was the influx of nasty Italians. Especially those from southern Italy. All you have to do is read any of the several books Lothrop Stoddard published through the 1910's and 1920's to understand the leverage gained by bringing the language of science to bear on racism. Stoddard was no fringe figure -- he was central to a powerful eugenics movement that helped push through the Immigration Act of 1924, which was aimed largely at non-Western Europeans:
The new law cut the quota for northern and western European countries by 29
percent, but slashed that for southern and eastern Europe by 87 percent. Italy's
quota, for example, was reduced from 42,057 to 3,845 persons.
Stoddard's own descriptors of these supposedly loathsome immigrants are found all through his work. Dig around.
However, at some point -- long before the term "PC" ever came into existence -- it became a bad thing in polite society to use terms like "wop" or "nigger." As a society, we began to see these terms as indicative of institutionalized and condoned racist thought. Likewise, the people who used such terms were understood to be either ignorant or racist or both.
PC is a term co-opted to counter anti-racism and movements to provide equal access in society. It is a way to turn the focus away from the bigots and onto false grounds of "thought police," "speech nazis," etc.
How many people remember the old apologist's saw that the Civil War was fought over States' Rights? Sure, technically it's correct to argue that the South was upset by the prospect that the North might get powerful enough to pass federal laws contravening their state laws, but only a moron would overlook the fact that these laws had to do with slavery. Slavery was the cause of the Civil War and the cause of every damn compromise from 1820 on. Hide behind the states' rights argument all you want, the fight was over whether our nation could continue to hold other human beings as domestic animals.
PC is bullshit.