The British say someone is on about something; Americans say going on, or going on and on. The first citation in the OED is from Rosamund Lehmann’s 1936 novel, Weather in Streets: “Marda’s always asking me why I don’t get a divorce… Last year she was always on about it.”
Welcome to NOOB-hood, bro.
- Kathryn Schulz (@kathrynschulz) writes on Twitter: “While I’m on about etymology (I’m always on about etymology): ‘adamant’ gets its root from ‘diamond’ — hard, unbreakable.”
- Kelly Dwyer on Yahoo Sports a couple of weeks ago: “I didn’t see a second of TNT’s Thursday night package, and didn’t hear what [basketball commentator Chris] Webber was on about.”
- “G. Funk”‘s comment on an article about professional wrestling on The Bleacher Report: “That’s why [the Ultimate Warrior] was the best. No one had a clue what he was on about, but everyone loved it.”
An early U.S. use came from the Rev. Al Sharpton, quoted in a 2002 New York Times article about a taped conversation he had with an undercover agent posing as a drug dealer: ”The guy had come to me. In the middle of conversation he started talking about how he could cut me in on a cocaine deal. I didn’t know what this guy was on about. I didn’t know if he was armed. I was scared, so I just nodded my head to everything he said and then he left.”
Always a groundbreaker, the Rev. is.
notoneoffbritishisms.com 
The British also say someone is “going on” or “going on and on”, possibly “about” something. (“On and on” suggests tediously.) When one says someone is “on about” something, the “going” is implied.
Other expression in the same vein : rattling on about or banging on about something …….
You can also “harp” or “bang” on about something.
I actually commented on this under “What It All Means” back in October. “on about” is definately a NOOB, no American would say it unless they were trying to sound British.
I predict that the wonderful British phrase “at it” (that is, engaging in sex, in one form or another) will migrate westward. At least I hope so.
Often heard as:
‘wot chew on abaht?’
Meaning:
‘what are you talking about?’
You can also ‘sound off ‘ about something, which is similar to go ‘on about’ or harp on about sth.
Or: “bashin’ me lug ‘oles”, “beatin’ me brains art”, *givin’ me grief”, “borin’ me shitless”? (North Britishers may require GoogleTranslate)