Meaning of "private gentleman" in "The Worst Crime in the World"?

Question

In "The Worst Crime in the World" by G. K. Chesterton, Father Brown and his friend, a lawyer, was talking to a rich old man who has lived alone in a castle for a long time, and who has just said that he will never talk to his son as long as he lives:

The lawyer said: “Why, what on earth has he — — ”

“I am a private gentleman,” said Musgrave, “as well as the custodian of a great inheritance. And my son did something so horrible that he has ceased to be — I will not say a gentleman — but even a human being. It is the worst crime in the world. Do you remember what Douglas said when Marmion, his guest, offered to shake hands with him?”

“Yes,” said Father Brown.

“‘My castles are my king’s alone, from turret to foundation stone,’” said Musgrave. “‘The hand of Douglas is his own.’”

I found that "private" may mean many things:

1. living alone: And he was living alone indeed as I said

2. made his wealth independently

3. prefer the privacy

I think that the third meaning is the most suitable in this context, isn't it?

And does "my king's" mean "mine", as I never met it before and can't find its meaning in any source?

Answer 1

A fourth position: I do not hold a position of public trust.

As an official, it would be his duty to deal with odious people. As a private individual, he has no such duty.

The reason why I think this likely is the anecdote about Douglas. His castle was a public trust -- the king had installed him and his family in it, in return for his services as an earl -- but his hand was his own and therefore he could keep it private.

(That is what he meant by "the king's alone" -- it was, in reality, only the property of the king, he was just entrusted with it. Bit of hyperbole in that, but a declaration of absolute loyalty.)