It is said that the terms originated in the UK when one side-the covenanters wore red around their necks to signify rebels in the Bishops Wars that preceded Cromwell. It eventually became a term towards Presbyterians in the lowlands of Scotland as well as the United States.
Hillbilly was supposed to be a term to signify Highland and other Scottish people who fought for William and Mary of Orange. Remember that Mary was the Granddaughter of the house of Stuart and they invited William into the fold to finalize the deal within the monarchy in the UK. He really wanted NOTHING to do with England AT ALL. As did many of the Germans who took that crown.
Pretty messed up hey? BTW, the name Windsor is not really the name of the present royalty. The name was changed during, or prior, to WW1.
Of course, the argument is that it all, redneck and hillbilly, originated in the USA.
My family is all Scotch-Irish. My understanding is that the term hillbilly came from the Scotsmen who lived in the highlands and their decendents that settled the Southeast USA. There are a lot of rednecks that are just plain white trash, but there are a lot of us that are self reliant and don’t want any government interference in our lives. My grandfather was a rancher and he taught me a lot about caring for animals and for family. I have passed that along to my adult children.
In 1917 George V saw a reference to himself as part of an ‘alien and uninspiring dynasty’. Reputedly he said “I may be uninspiring, but I'll be damned if there is anything alien about me’. A month or so afterward the Royal Assent was given to change the house name from Saxe-Coburg-Gotha to the resoundingly British Windsor.