I’m on about

[Maybo]

What does “I’m on about” mean?

 

[Tarheel]

That's the one she's talking about.

 

[emsr2d2]

"on about" or "verb + on about" means that it's the subject being discussed (written or spoken).

That's the car I'm on about = That's the car I'm talking/writing about.
What are you on about? = What are you talking about? = I don't understand what you're saying/writing.
He goes on about his girlfriend all the time! = He talks about his girlfriend all the time.

 

[Tarheel]

If I would use that phrase with an American they wouldn't know what I was on about.

 

[emsr2d2]

If I would use that phrase with an American they wouldn't know what I was on about.

That shows it's an AmE/BrE difference. The start of your post is an example of another one. In BrE, we don't use "If I would use ...". We say "If I used that phrase, ...".

 

[Maybo]

Can we use that phrase in formal writing?

 

[emsr2d2]

Can we use that phrase in formal writing?

I wouldn't.

 

[jutfrank]

Can we use that phrase in formal writing?

Absolutely not!