be/go/keep on at somebody
be on at
1. To be televised at a particular time (which follows "at"). The movie is on at 8, so I'm going to start making the popcorn now.
2. To nag someone about something. In this usage, "at" is followed by a noun or pronoun. Ugh, my mom is on at me all the time about my grades, but I just have no interest in the classes I'm taking this semester.
See also: on
keep (going) on at someone or something
to continue to complain about or scold someone or a group. Don't keep going on at him. Give him the dickens and be done with it. Please don't keep on at the committee. They did the best they could.