be on at someone

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

be on at someone

nag or grumble at someone. British informal
See also: on, someone