There is a common Yiddish phrase used to describe an arbitrary reasoning that is not based on the sources: "Boich sevarah" -- literally, "stomach reasoning." What is the exact meaning of this phrase, and for how long has it been used? [This question was inspired by two pieces in Teshuvos Chasam Sofer (YD 129 and 353) that reject someone's "סברת הכרס".]
A search on Hebrewbooks yields the following earlier (16th-century) uses of the Hebrew סברת הכרס:
Radvaz, teshuvah 1463: זו סברת הכרס היא
Yam Shel Shlomo, Gittin 4:28: וכל אחד עושה לו סברת הכרס כמו שיחפוץ
...and I'm sure there are others too.
Considering that Radvaz lived nowhere near any Yiddish speakers, I doubt that the Hebrew is a calque of the Yiddish; I'd guess it's the other way around. But I don't know who originated it.
The meaning seems pretty straightforward: like Ernest said, it's something like "a gut feeling" rather than an idea developed through proper cerebral thinking.