When I use the word "Flatlander" I'm not talking about a country music band from Lubbock. Nor am I speaking of a book by Larry Nivens. And, I am certainly not using the expression as a derogatory comment about country folks.
I'm talking about folks who live on the prairie, or the flatlands.
I grew up in mountainous country, though the mountains near my home were desert mountains with bare rock, dirt, and desert plants. Despite that, if you took a short drive of 75 miles or so, you could find yourself in the tall timber and high mountains of Arizona.