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When friends trade who is the fat one. This is due to the one that was fat getting healthy at the same time that the one that was healthy got fat, this exchange is called a fat swap.
"I remember the two of you, didn't he used to be the fat one? What did you do, a fat swap?"

"Jane used to be fly as hell, now she looks like a mammoth"
"Oh she did a fat swap with Latisha though, now Latisha is damn fine"
by TheDogman January 07, 2014