(Cap-pay). Hugues, the founder of the French monarchy, was surnamed Capetus (clothed with a capot or monk's hood), because he always wore a clerical costume as abbot of St. Martin de Tours. This was considered the family name of the kings of France; hence, Louis XVI. was arraigned before the National Convention under the name of Louis Capet.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894