The story
From the song Jump Jim Crow (c. 1830) which mockingly portrayed African-Americans. It became a generic term for African-Americans in the early- to mid-19th century.
Etymology from Wiktionary, CC BY-SA 4.0.
Jim Crow law
Today it means: Any law that racially segregated public facilities and which was enacted in Southern and border states in the United States between 1876 and 1965. Full definition →