What does Uncle Jack's reference to Lord Melbourne in To Kill a Mockingbird mean?

In To Kill A Mockingbird, Uncle Jack relates a story about Lord Melbourne to Scout. In history, Lord Melbourne was Queen Victoria's first Prime Minister. Melbourne had the reputation for being something of a ladies' man.

This is the story Uncle Jack tries to share with Scout when Scout asked him what was a whore-lady? Uncle Jack is caught by surprise at Scout's question. He tries to share a story about a character who is morally in question. Scout says Uncle Jack's story made no sense:

Uncle Jack plunged into another long tale about an old Prime Minister who sat in the House of Commons and blew feathers in the air and tried to keep them there
when all about him men were losing their heads. I guess he was trying to answer my question, but he made no sense whatsoever.

No doubt, Uncle Jack is caught off guard when Scout asks the following question:

'What’s a whore-lady?'

Uncle Jack does not come right out and answer Scout's question. For this reason, Atticus tells him that he should be more direct:

Jack! When a child asks you something, answer him, for goodness' sake. But don't make a production of it. Children are children, but they can spot an evasion quicker than adults, and evasion simply muddles 'em.

Clearly, Scout's direct questions can be challenging. It is not surprising that Uncle Jack did not know exactly how to answer Scout's "whore-lady" question. While he was trying to answer her question, Scout said "he made no sense whatsoever."  

In To Kill a Mockingbird, why does Uncle Jack tell Scout about Lord Melbourne?

In chapter nine, Uncle Jack visits his brother's family to celebrate Christmas and is forced to reprimand Scout for punching her cousin in the face. Later that evening, Uncle Jack has an enlightening conversation with Scout about what transpired, and he apologizes for jumping to conclusions after she tells him what Francis called Atticus. Uncle Jack then leads Scout into the bathroom, where he proceeds to bandage her knuckles. While he is dressing Scout's minor wounds, she asks him, "What's a whore-lady?" (Lee, 89). Uncle Jack is immediately caught off guard by her explicit question and attempts to dance around the subject by telling her an outrageous, entertaining story concerning Lord Melbourne, whose life was quite colorful. His story about Lord Melbourne does not answer Scout's specific question and is simply used to distract her from the explicit topic.

Later that night, Scout overhears Uncle Jack discussing their conversation with Atticus. When Uncle Jack tells Atticus that he avoided answering Scout's question about the meaning of the term "whore-lady," Atticus tells his brother,

Jack! When a child asks you something, answer him, for goodness’ sake. But don’t make a production of it. Children are children, but they can spot an evasion quicker than adults, and evasion simply muddles ‘em. (Lee, 90)