Oh, okay, I've seen it both ways in previous salon posts and just picked one at semi-random.

My search function tells me Selena has said "Butcher Cumberland" 3 times and "Butcher of Cumberland" 1 time. I only have memories of seeing "Butcher Cumberland" in my reading, though it's possible reputable sources do use "Butcher of Cumberland" and I either haven't encountered them or my memory is failing me.

(But wouldn't the proper analogy be "Detective of Midgard" rather than "Detective of Mildred"? ;) )

Ha! I knew you were going to say that. ;) They feel different to me, because the "of" in "Duke of X" isn't the same as the "of" in "Mildred of X": one is "[title] [over] [domain]" and the other "[person] [from] [place]". My impression has always been that "Duke of Cumberland" was like "Prince of Wales," in the sense that it was granted to members of the royal family who were of the appropriate rank and needed a title to go with their rank. I don't think William was associated with Cumberland much more than Frederick was with Wales. I mean, maybe he was granted an estate and some income from the region? but I don't think he spent time there when not crushing a Jacobite rebellion. Saying "of" to mean "from" would feel weird to me in his case.

Wikipedia says:

Duke of Cumberland is a peerage title that was conferred upon junior members of the British Royal Family, named after the historic county of Cumberland.

The dukedom was created in the Peerage of England in 1644 for Prince Rupert of the Rhine, nephew of King Charles I. When he died without male heirs, the title was created again in the Peerage of England in 1689 for Prince George of Denmark, husband of Princess Anne, younger daughter of King James II. He also died without heirs, in 1708. Neither of these men, however, was usually known by his peerage title.

The third creation, in the Peerage of Great Britain, was for Prince William, the third son of King George II. Other titles granted to Prince William were Marquess of Berkhampstead, Earl of Kennington, Viscount Trematon and Baron Alderney. Since the Prince died unmarried and without children, his titles became extinct at his death.

I agree that if we're talking about like the Duke of Brunswick, I would have a different reaction to "of", since the dukes actually lived there and/or came from there, in addition to having the title. Though that's almost entirely because I would be able to say "Ferdinand of Brunswick" (just like Rupert of the Rhine, George of Denmark) in a way that "William of Cumberland," without the "Duke".

I am open to counterevidence, though! I'm not a hundred percent sure here. (Except that I've definitely seen "Butcher Cumberland" way more than "Butcher of Cumberland," even if someone proves the latter does exist.)