"Master Juba" was the stage name of William H. Lane (ca1825-1852), said to have been the greatest dancer of the 19th C. minstrel stage. He was an African American, "lionized" in London when he played there with Pell's Serenaders in 1849.

His cue appears to have been, "Can you dance, Juba?"

Hence, "to dance juba."