I did some research on this awhile ago and after an hour or so all I knew was that nobody agrees on whether or not it's an acronym.
My Grandfather used to call pennies "coppers" hich made sense to me since pennies were made of copper (in those days!) When I visited England in the 1970s I saw the British penny which was quite large and copper!
The word copper was interchangeable with policeman, too.
Go figure.