Oyster-shelling, black-balling, or expelling. Clisthenes gave the people of Attica the power of removing from the state, without making a definite charge, any leader of the people likely to subvert the government. Each citizen wrote his vote on an earthen. ware table (ostracon), whence the term.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894