The feeling of lethargy and simultaneous content and warmth you get from eating a big bowl of warm soup- especially exaggerated on a cold day. Sorta like what happens to most folks on Thanksgiving, only warm soup belly can happen all year long.
"Gee, I'd love to go sledding with you guys, but that bowl of chowder I ate has given me a case of warm soup belly. I think I'm just gonna sit here and veg."
"I got that warm soup belly and now I'm ready for a nap."