I agree with Greybeard on it. Goes way back and started with "Behind the eight ball". In Eight Ball, you cannot use the 8 as a combination to make your balls. If you are stuck behind it you are in a very bad position with little way out. Meaning a very probable missed shot will leave your opponent withe upper hand and you will lose. Loser and the symbol go together. Later in military is was associated the same way.
