Someone eating or aquiring comically large and/or frequent amounts of food, or appearing to do so.
Must be said with a south-west english accent (Cornwall, Devon, etc.)
Only works with the 'You' at the front, and only happens with 'You' and 'Bif' on one note, and 'fat'and 'fer' on a higher one.
There's no real emphasis anywhere, and it's deadpan despite the humourous intent.
The hand with which you have rubbed the head of you're unwashed penis to climax is known as the "BiffedHawn" - e.g you have Biffed yourself hand-wise, self abuse. It now has a vomit inducing scent that should be washed more thoroughly than a coughed in hand.