It's defined as:
"A person whose roots are Appalachian, but who has gone on to become educated while still appreciating his or her mountain roots. Coined by Jane Hicks in the poem "How We Became Cosmic Possums" and referred to in Sharyn McCrumbs's novel, The Songcatcher.
JohnBoy of the old TV show, The Waltons was the first cosmic possum."
(Not to quibble, but wouldn't cosmopolitan-possum be more accurate?)