The expression “eight ball” for the black pool ball numbered 8 is from the early 1900s, and the expression “behind the eight ball,” meaning “in a highly disadvantageous position,” entered American English. The expression may have come from a pool game in which players attempt to pocket the numbered balls in order – with the eight ball to be pocketed last. If the player hits the eight ball first, he is penalised; if he pockets the eight ball, he loses the game.
“A piece of eight” is an old Spanish silver dollar, or peso, which once had the value of eight Reales and was therefore stamped with a large figure 8. In Colonial America, the piece of eight became legal tender, and it remained so till the Civil War. Its value was almost the same as that of the United States dollar.
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