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1.
A situation in which you gain quite a bit of weight without realizing it.
Having not seen him for a few months, Heather says to Barry, "Gee weez, Baz, I almost didn't recognize you!"

"Yeah, I know, I’ve had a serious case of fat creep, hadn’t jumped on a scale for a few weeks and boy did I get a real shocker this morning, I’m going to download an App for Fat Creep to melt this blubber for good.”
by Girl Banker September 23, 2011