The Duke of York was Bishop of Osnaburg Not prelate, but sovereign-bishop. By the treaty of Westphalia, in 1648, it was decreed that the ancient bishop should be vested alternately in a Catholic bishop and a Protestant prince of the House of Luneburg. Frederick, Duke of York, was the last sovereign-bishop of Osnaburg. In 1803 the district was attached to Hanover, and it now forms part of the kingdom of Prussia.
Osnaburg A kind of coarse linen made of flax and tow, originally imported from Osnaburg.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894