be on somebody's back

be on (one's) back

To be 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.
See also: back, on

be on somebody’s ˈback

(informal) annoy or criticize somebody a lot: My new boss is on my back all the time, saying I have to work harder.
See also: back, on