on (one's) back
Constantly nagging one about something. Ugh, my mom is on my back all the time about my grades, but I just have no interest in the classes I'm taking this semester.
on someone's back
Also, on someone's case. See under off someone's back.
on someone's back
If someone is on your back, they are criticizing you a lot or trying to persuade you to do something. The crowd aren't forgiving — as soon as you make a mistake they're on your back. If they find out you're weak, they're on your back straight away. Compare with get off someone's back.
on your back
in bed recovering from an injury or illness. on (someone's) back
Persistently nagging or urging someone to do something.