When you reach the level of fatness that your stomach starts to take on the same firm-yet-lumpy consistency of a cooked sausage.
The stage somewhere between a muffin top and losing Cheetos between your rolls.
Reference: Jurassic 5 - Sausage Gut
Person1: Check this out, my sausage gut is starting to mature. When I squeeze my rolls they're all firm and lumpy. If I keep it up with the eating contests, cheeze whiz snorkel, and twinkie sepository diet, I'll make it on The Biggest Loser in no time.
Person2: ::ralf::