The polite plural is common enough in European languages; there are French dialects (in Canada, at least) where a good rule of thumb is "don't use tu unless you're sleeping with that person or are their parent". With the second person singular restricted to intimate use, it's not hard to imagine it falling away.
Well, for linguists "thou" and "you" is the same word.
Not just in that they mean the same, but in the same sense that in evolution X1 is said to be an ancestor species to X2.
"Thou" and "You", is the same word passed through a process of changes in pronunciation and spelling during the years. Linguists keep track of those kinds of things.
Theres some idioms that'll never die even if no one understands the history. "Holier than thou". Much like "Your name is Mudd" sometimes kinda horrifies people who don't know the story of Abe Lincoln and Dr Mudd. I've met people who claim to not know the history of "cotton pickin hands" or even acknowledge its racial background, which seems hard to believe, then again cotton hasn't been hand harvested in a couple generations locally so its possible the kids were telling the truth about not knowing what they're saying...
The Wikipedia article on thou explains this briefly in the introductory paragraphs: https://en.wikipedia.org/wiki/Thou
It's probably just an intentionally quaint pseudo-folk saying invented by someone in Burbank.
LOL no, assuming you young folks pretending not to understand are serious. As a hint take a look at the skin color and local social hierarchy position of the people picking cotton back when that was still done by hand, by property or former property, as they used to say. It was basically used in place of the "N-word" when it was just barely not socially acceptable to use the "N-word" in that situation.