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1.
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!

Fat Person: 'Ha! look at that bitch Lucy over there'
Me: Shut the fuck up you fat knacker!
 
2.
someone who is so fucking fat they can't see their feet, walking from the sofa to the fridge is enough to knacker them out
holy fucking christ look at the size of that fat knacker going to mcdonalds, we'd better order first or there will be nothing left.
by Big man March 13, 2003