Pool or pocket billiards term meaning: You are about to lose a game of eight ball because the opposing player is shooting the eight ball to win the game, and you are there standing or sitting behind it.
Has come to modern usage meaning you are in a bad situation or about to lose....or lost someone or something.
That new guy who was a math wiz left today: now we're really behind the eight ball.
I've been behind the eight ball ever since she left.
They put us behind the eight ball when they drafted that kid from Pacoima in the first round!