Nothing flat

I told my son that we would be there in "nothing flat."
He asked when and where "nothing flat" originated?
Any ideas?

Perhaps it comes from the meaning in Webster;
Flat; 7. Clear; unmistakable; peremptory; absolute; positive; downright.

Flat burglary as ever was committed. --Shak.
A great tobacco taker too, -- that's flat. --Marston.

If two minutes flat is just two minutes, then nothing flat would be no time or distance at all.

Indeed, people--Americans, at least--say "We'll be there in no time."