1. Have your back to/against the wall
- to have very serious problems that limit the ways in which you can act
“He owes money to everyone - he really has his back to the wall now”
2. Behind someone's back
- if you do something behind someone's back, you do it without them knowing, in a way that is unfair
“I dread to think what they say about me behind my back”
3. Break your back
- to work extremely hard
“He broke his back to get the project done on time”
4. Get off someone's back
- used to tell someone to stop criticizing you:
“Why don't you get off my back! I'm doing my best”
5. You scratch my back and I'll scratch yours
- used to tell someone that if they help you, you will help them
“The kids in that class have an “if you scratch my back, I’ll scratch yours” attitude and cheat plentiful during examinations”