To be in a challenging situation; to be at a disadvantage. An "eight ball" is one of the numbered balls used in the game of pool, and the last one supposed to be hit into a pocket. If the eight ball is accidentally hit into a pocket prematurely, the player will lose, and it therefore needs to be avoided. If we don't leave early tomorrow, we'll be behind the eight ball, and I doubt we'll reach the cabin by dusk. Because I got the flu, I'm behind the eight ball with my Christmas shopping.