Be on about something - Idioms by The Free Dictionary
be on about something
be on about (something)
To be 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 ˈon about something (informal) talk about something; mean something: I didn’t know what he was on about. It didn’t make sense.
be/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.