A fat pie is an overweight female, generally with some redeeming features (i.e cracking tits or nice face) who is quite tasty, although the term can be used loosely to describe any fatties.
Would typically be do-able after about 4 pints.
G'ham: "Check out that jailbait!"
Dave: "IDK man, she's a bit of a fat pie.."
G'ham: "True, maybe I'd wrap it first."