Doctor: I'm afraid your son has "The knack"
Mother: Can he lead a normal life??
Doctor: No. He'll be an engineer.
Mother: Oh no...
Doctor: There there.
contrary to popular belief the term 'fat knacker' is quite often used to insult someone who is not actually fat. But who is stuffing their amply sized gob full of something a fat person would eat.
Implies that they get knackered whilst eating hence the term fat knacker.
Also can be used to describe someone who is just insanely fat and gets tired shuffling to the fridge.
(note: i have absolutely nothing against fat people, the term fat knacker would only be used (in my case) to describe someone fat and a total ass wipe)
WTF! you're going back for another hamburger? you better hurry up and eat that then you fat knacker!