It is funny because it is a pun, a play on words. In the old time black and white movies, there was a famous black man character named Stepin Fetchit who would walk very slowly and do everything so slowly, you could not believe it. He was always a servant of some kind.
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In the early thirties, there was a black actor named Stepin Fetchit that became famous. He played the stereotype African American known in Hollywood then. A caricature of what African Americans were believed to have been like.