Fatbag is another word for a shopping bag stuffed with - often discounted - products. A fatbag is stuffed to a level where it almost breaks, thus looking like a fatbag.
He carried a fatbag out of the shop as he had made many good bargains.
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A fatbagger is a bargain hunter buying large amounts of products (often expensive brands and products) when they are on sale. From the meaning of filling a big shopping bag with products making the bag a fat bag.
As a real fatbagger he enjoyed shopping big when the price was right