When someone is "behind the eight-ball" he is in a difficult, if not impossible situation.
   The expression comes from the game of Kelly pool. Pool or  pocket billiards, is a table game played with fifteen numbered balls and a cue  ball. Players strike the cue ball with the end of a cue, or stick, sending it into one or more of  the numbered balls. The idea is knock each of the numbered balls into one of the six pockets around the edge of the pool table.
   In  one  kind  of  pool  game  called eight-ball, players try to put the numbered balls into the pockets, with  the  eight-ball  before  all the other balls are in the pockets. A player can put his opponent in poor position by leaving the cue ball behind the eight-ball. The eight ball blocks his opponent from a shot at any of the other balls.
   So, if  you are  behind the  eight-ball, your position is bad. There is almost no way out of the situation. You are in trouble.

　　说一个人“藏在八号球后面”，就是说他处于困难的即使不是没有希望境地。
　　这个说法出自一种台球游戏。台球，也叫落袋弹子，是一种在台桌上玩的游戏，共有十五个编了号的球和一个主球。玩球的人用球杆击打主球，用它去撞其他的编了号的球，目的是将这些编号的球一个个打进台桌边上六个袋子里去。
　　有一种叫台球游戏叫“八号球戏”：玩球的人要设法先将其他编号的球都打进袋子里，最后才把八号球打进去。如果还有别的球没有进袋而玩球的人却不小心把八号球打了进去，那他就输了。这样，谁要是能把主球藏在八号球后面，他的对手可就落入困难的境地：八号球挡住了主球，使他无法打其他的球。
　　因此，如果你要的东西藏在八号球后面，你的处境就很糟糕，简直没法逃脱，困难重重。