It's from AMOS & ANDY, a show from the '50s that is way too politically incorrect to ever air on commercial television again. But this joke killed me when I first heard it, and makes me laugh every time. And don't worry. It's not racist. Just cruel.
I often talk about how important the set-up is in constructing jokes. This is a perfect example. It's the set up that makes this gag work. At least for me.
