"White dog" is common slang for moonshine; it embodies the crisp, simple, nature of whiskey fresh from the still. Without first having developed a tasty, unaged spirit that can stand on its own, a distiller's complex, barrel-aged spirit has no foundation on which to stand.
All great spirits begin as "white dog", but it's only through time and dedication that they develop into the complex and flavorful aged expressions often associated with distilled spirits.