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1.
When an individual wears many layers of clothing to stay warm from the winter frost and ends up looking bulky.
Wil: "Damnnn..did Sarah gain weight?"

Kate: "No, that's just her sweater fat."
by katexblonde December 07, 2010
 
2.
when you buy a sweater that is too big for your actual size
thus giving you a lot of extra cloth
producing the effect that you have a lot of fat

when you actually have sweater fat
Person 1: "Dude go do some jump ropes"
Person 2: " ugh..it's annoying with all this sweater fat"
by Our Twig November 18, 2010