The labored sigh that fat people make whenever they sit down, stand up, pick up somethingheavy, climb a short flight of stairs, reach for something on a high shelf, etc.
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.”