be on about (something)
To speak about something tediously and at great length. Primarily heard in UK. He just kept rambling about literary metaphors and imagery. I didn't know what he was on about! She's always on about some new show she has started watching.
be on about
talk about tediously and at length. British informalbe/go/keep ˈon about something
(informal, disapproving) keep talking about the same thing so that people become bored or annoyed: What’s she on about now?Don’t keep on about your terrible journey. It’s so boring.