Is on their back - Idioms by The Free Dictionary
be on (one's) back (redirected from is on their 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.
be on (or get off) someone's back nag (or stop nagging) someone. informal
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.