I know the meaning, but it's not sure there are no connections with pooping... In French, we say a "une m*rde arrive" for a bad stuff happening.
This expression could be a corruption of "It happens", but it could be also related to the stercus. It's not impossible (like in the Forrest Gump movie)
Dung was also used in remedies for baldness in the ancient world. The 5th century African writer, Cassius Felix, provides us with a recipe for a hair generating ointment:
«Stercus muris cum aceto tritum et illinitum capillos educit».
[The dung of a mouse, ground and rubbed with vinegar, produces [new] hair].
—Cassius Felix, De medicina; Ad alopiciam.
As a Romance language person, "to sit" used for things, even if I know it's correct, is always a bit weird to my mind.
I was convinced that "to sit" couldn't be used for things in Latin, but only for humans and animals, but the Gaffiot told me I was wrong:
Sedere first meaning: to sit (human, animals) on a chair.
But can mean to remain (for humans or things, used with fog in a sentence, etc...)
But I still doubt it was used like the verb "to sit" in english, to mean: to be there, to be placed there, for things.
In French, Sedere gave:
Asseoir meaning to sit on a chair, (but more rarely figuratively, to establish something.)
Seoir, (conj. il sied), meaning to be located (archaism). And also: to be suitable.
Situer/situation, that became later situate/situation) in English, same meaning.
And of course, un Site, from Latin Situs (location, place).
And the adjective "Sis" (from Latin sessus "to be seat-ed"), from the verb Seoir: to be located.
The "sis" meaning is close from the Latin meaning here, and is from the same root, it's probably a hint, but I'm still not convinced, because of lack the dictionaries examples, meaning something else than "to remain". Someone could help me?