It's a type of coke, of course! It's a Southern thing. ;)

(I suspect that it's a byproduct of Coke having originated in Atlanta so way, way back once upon a time they literally all were Coke products in the South. It's a bit like using the name "Linoleum" for sheet vinyl flooring products (even though they aren't technically linoleum and 'Linoleum' is actually a trademarked brand name) or 'Kleenex' for tissues or 'Dryvit' for EIFS, etc.)