a new one on

that's a new one on me

That is something I was not aware about or did not know. A: "Yeah, Jeff and Anthony have been going out for a few months now." B: "That's a new one on me. I thought they were just friends." You aren't allowed to dump your grass clippings here? Well, that's a new one on me.
See also: new, on, one

a new one on (me, him, etc.)

an account, idea, or joke not previously encountered by me, him, etc. informal
See also: new, on, one