John: How could you sell my mother’s necklace? It’s been in my family for five generations!
Marcia: My back was to the wall! If I hadn’t sold it, we would have lost the house!

You say your back is to the wall when you are in a desperate situation. Imagine you have been attacked by zombies. You will be okay so long as you can run away, but if your back is against the wall and the zombies are coming toward you, you are trapped and have nowhere to run.