"I must just go and see the Duc d'Aumale"

Prince Henri, Duke of Aumale
Public DomainPrince Henri, Duke of Aumale
Prince Henri, Duc d'Aumale (1822-1897) was the youngest son of Louis-Phillipe, the last king of France, and an advocate of constitutional monarchy. He was ridiculously wealthy (worth about £200 million in today's money) and amassed a collection of manuscripts and books, as well as an art collection which he bequeathed to the state. There is no direct reason why Cottard uses him as a lavatorial euphemism.