A crazy, reckless person. Furthermore, a lighthearted term used to describe a mix between an idiot and a daredevil (either for insult and, sometimes, endearment).
Like the literal black ball used in a game of pool, an Eight Ball is a living, breathing symbol of both good and bad luck.
Popularized by U.S. Navy and
Marine Corps T.V. shows "McHale's Navy" (1962-1966) and "Baa Baa
Black Sheep" a.k.a. "
Black Sheep Squadron" (1976-1978).