the ins and outs of a ducks bum

Does anyone know the origin of the phrase "the ins and outs of a duck's bum" and its exact meaning.

"Bum" is a chiefly British, sometimes vulgar word for "buttocks".
So someone who knows "the ins and outs of a ducks bum" is someone who knows all the minute details of something or of a process.
This board is known to be frequented sometimes by people from that part of the world so, hopefully, one of them will step up and give us some more detail.