Thomas Dartmouth "Daddy" Rice (1808-1860) was an aspiring white actor from New York who gained notoriety in the 1830s when he delivered a performance in blackface before a theater audience in Pittsburgh, Pennsylvania. Blackface was makeup, initially burnt cork or coal, applied to the faces and limbs of white performers to give them an exaggeratedly black appearance—far blacker than any real African-American. Rice's "Jim Crow" song and dance won rave reviews from white audiences in major cities throughout the North.
By the early 1900s, "Jim Crow"—the name of Thomas "Daddy" Rice's blackface character—described the institutional segregation in place in the American South. To this day, it is unclear why this elaborate system of racial separation became synonymous with Rice's minstrel routine. It's possible that whites in the early twentieth century, still preferring the image of the jovial, gimpy slave to more sober and truthful representations of blacks, found the phrase useful for referring to such laws.