If you’re “behind the 8-ball,” you’re at a disadvantage, in a losing position. According to a 1931 issue of the New York Times, the phrase means “in a tight spot.”
It is often assumed that the expression derives from the inability to use the 8-ball in a combination in the game of eight-ball—if the cue ball is directly behind the 8-ball, a player has no direct shot route. However, numerous billiards-specific and etymological publications state that the expression derives instead from kelly pool, an early version of the now popular eight-ball.
The phrase is traceable back to at least 1919, but the form of game that became “eight-ball” was not described by that name, and its rules were not published in any official rule book, until after 1940.
An origin of the phrase independent of any particular game’s rule, instead depending on a property of the 8-ball itself, is offered by The National Billiard Association, which was the governing body of American billiards from 1921 to 1941: “It is generally conceded that the 8-ball is the most difficult for the player to see clearly in the execution of his shot. This, because it is black, naturally the edges of the ball, or in fact any part of the ball, do not stand out as clearly as colored balls. Therefore, professional players, if possible, avoid being forced to play the 8-ball from a difficult position or with the cue ball a long distance from the 8-ball because it is more difficult to see clearly.”
In the game of snooker, the roughly equivalent idiom is “snookered,” and it too has entered the language to mean much the same as “behind the 8-ball.”
Behind The Eight Ball was used in the title of a biography of “Minnesota Fats'” — the stage name of the pool player Rudolph Wanderone. But Wanderone was by all accounts a much better self-publicist than he was a pool player. He played some televised demonstration matches against Willie Mosconi, who beat him easily.
I’ve always wanted to learn to play pool. But so far, as close as I’ve come is doing the side stroke in the local recreation center swimming pool. Which means I’m often all wet, but not snookered.