What does "back to the wall" mean?

Also, back against the wall. In a hard-pressed situation; also, without any way of escape. For example, In the closing few minutes, our team had its back to the wall but continued to fight gallantly, or The bank has him with his back to the wall; he'll have to pay up now. This term was used originally for a military force that is making a last stand. [First half of 1500s]