Too bad you're not interested in songs that are intentionally offensive, because they're much easier to find: they include a large chunk of what has passed for "folk music" since the 1960s. Of course, the people who sing them usually don't think of them as offensive, just "factual" and or "speaking truth to power."

By innocently politically incorrect, do you want to include words that have changed meanings, such as the line from Pack up Your Troubles ("While you've a lucifer to light your fag")?

Then there's the Loo Song which as originally written I believe starts out, I was born in Arkansas, my mammy was a squaw.