Estar en aprietos; estar en el hoyo; con la soga al cuello
To be behind the eight ball means that you are in trouble, in a difficult position or in a situation in which you have little hope for winning. Other similar expressions are: “In a pretty kettle of fish,” “in a pickle,” “in the dog house,” and “in hot water.”
If you have played pool or billiards, you will easily understand why people use this expression. There is a pool game in which you have to hit the balls into a pocket one by one in numerical order. But the eight ball has to be pocketed at last. So, if the eight ball is bettween the white ball and the ball that you want to hit, then, you are in trouble and you will have to think of a clever move to keep playing or you will lose your turn.