1. be in a very difficult and limiting situation
with the deadline approaching, my back is against the wall
This expression does not just refer to a difficult situation, it refers to one in which your options to escape are limited. It also implies an inability to retreat, so a person with their 'back against the wall' would need to face their problems and fight and work hard to avoid failure or defeat. It is common in both a social and professional context, and is similar to the phrase 'between a rock and a hard place.'