Here I think this is right.
quote:
EIGHTBALL'Eightball' is a highly uncomplimentary slang word standing for a rather objectionable character ('gold brick', 'goof'). The word owes its origin to the billiard game where the number 8 ball has some restrictive associations which make it an object of disapproval. Namely, in a pocket billiard game, popular in the USA, the player loses if he directs the eight ball into the pocket. Hence, the phrase 'be behind the eightball' (in trouble).