"Jim Crow" was an antebellum character in a minstrel show. A white man (Tom "Daddy" Rice) - made up as a black man - incorporated a character called "Jim Crow" into his show in 1832. Jim Crow sang a song to this music (You will need Real Audio):

"Weel about and turn about
And do jis so,
Eb'ry time I weel about
And jump Jim Crow."

Soon the term "Jim Crow" became a euphemism for "Negro." Soon the term "Jim Crow Laws" became a euphemism for legal segregation.