The idiomatic English phrase “behind the eight ball” is used to indicate that someone or some group is in a bad situation, or a challenging circumstance that is not easy to resolve. This generally negative phrase can be reduced to more technical and shorter synonymous phrases such as "in trouble." English speakers understand the use of “behind the eight ball” as a more colorful way to describe a challenge or negative condition.
Although details on the origin of the phrase differ, most agree that “behind the eight ball” comes from the game alternately called pool or billiards. In what is arguably the most common type of billiards game, especially in many English-speaking countries, the eight ball has particular significance. Pool or billiards involves using a set of numbered balls, all of the same size and weight, but painted in different colors and patterns. One of them, usually a solid black ball, is numbered “eight.”
The common type of billiards game that led to the use of the idiomatic phrase is often called “eight ball pool.” In this game, players try to hit all of a set of balls into various pockets in a horizontal table, using long sticks, or cues. The eight ball is supposed to be hit last, and putting the ball into a pocket early results in losing the game.
Because of the rules of the game as stated above, players have to take particular trouble with the eight ball. When the eight ball stands in the way of a desired shot, it makes it hard for the individual player to succeed. This is the problematic situation that leads to the metaphorical use of the phrase “behind the eight ball.”
With this underlying metaphor, where the literal eight ball represents a challenging puzzle or dilemma, it would seem that the idiomatic phrase “behind the ball” would be used for something that also constitutes a particular mental challenge. Although this is sometimes the case, English speakers also now use the phrase for a more general problem associated with procrastination, such as being “behind the eight ball” at work, which can mean generally being behind on work, rather than contemplating a specific problem. In these sorts of uses, speakers use “behind the eight ball” is a phrase that’s generally synonymous with “under pressure.”