Serious question for any etymologists:

When did Americans stop being able to tell their backsides from their donkeys?

A 1900 version of Merriam-Webster had two definitions of ass: donkey or stupid person (both uses are present in Shakespeare, and probably earlier). And arse was also defined, correctly.

But at some point between then and when I was born, Americans started using ass to mean arse.

Was it the result of some censorship thing where arse was deemed offensive but ass wasn't?