> I am used to the word "stuhl" too. Which means both "stool" and also "chair"

Stool means both stool (a backless chair, usually thought of as wooden with three or four fixed legs but the lack of back is the defining feature and those made of other materials and/or with more legs are still called stools) and stool (solid excrement, as in “stool sample”) in English too.

IIRC the something-to-sit-on meaning came directly from one of English's Germanic influences, we just use it for a different/specific kind of seat, and the association with excrement came either from toilet seats (privy stools) generally or the royal position “groom of the stool” (where it meant cleaning both the privy stool and the royal backside that made that need cleaning).

I wonder if the German language took the second meaning back from English later, or if they both developed at the same time for the same reason at a point when there was shared influence between English and German royal courts.

> So, what's an "ap" then?

In this context my mind went to application. StoolAp sounds like a digestive tracking “health management” tool someone with a scatological bent might have on their phone!

I had to look this very interesting word up now. And it is spread all around the Germanic languages and is directly derived from the verb "to stand" (en), "stehen" (de) or "at stå" (dk). Which is funny, because you sit on something that stands. :-)

> I wonder if the German language took the second meaning back from English later, or if they both developed at the same time for the same reason at a point when there was shared influence between English and German royal courts.

I would bet on the second one!