A girl with a fair bit extra round the waist but DD's or bigger. Not very attractive but you probably would after 6 pints. Gerald's favourite type of girl.
Dave: Look at that fat pie over there Jezz!
Jezz: Oh, I just jizzed in my pants.
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."