Green’s Dictionary of Slang

interested in, talking about.

1956		J. Curtis Look Long Upon a Monkey 189: What I’m on about’s he having it with the bird.
1962		R. Cook Crust on its Uppers 48: Don’t know what you’re on about.
1970		C. Brown Down All the Days 174: What are you on about?
1986		F. Taylor Auf Wiedersehen Pet Two 223: ‘What’s he on about?’ said Oz, bewildered.
1999		Hip-Hop Connection Aug. 20: We haven’t a clue what they were on about but it sounded good.
2000		J.J. Connolly Layer Cake 192: I don’t know what the fuck you’re on about.