Fiber basically regulates water content in the bowels. It’s hydrophilic, so it “soaks up” excess water if that’s the problem and “retains” water if the lack of it is a problem. Either way you should get larger and better formed stools. Fiber helps, but it’s secondary to the real issue.
I’ve never heard the expression in that form.
I have heard that an idea “doesn’t float,” comparing it to a boat which makes sense. Maybe some people have inserted the word shit to simply mean “stuff,” making it a mixed metaphor?
snort
I think we’re forgetting neutral buoyancy. If the poo sinks like a stone, you’re not getting enough fiber, but if it floats on the surface, you aren’t absorbing properly.
I just used the phrase and saw your question. You have it a tad incorrect. The way I use it is “That shit don’t flush” and is similar to the idiom used in the South “That dog don’t hunt”.
Someone just handed me an excuse that was so unbelievable that it just sat there and stank up the room…as in…that shit don’t flush, honey. It just sits there and stinks.
Hope that helps.
It can certainly fly…especially when it hits the fan.