> Was it originally pure German?

Seems likely. The verb, as I wrote, is very common; I can't imagine that the noun would not have been used in German, what with the German's love for Nomen - I have no proof, but. The uses I found are mostly "misuses" where 'Vermeidung' would have fit better. S'all the same root though, anyway. A traditional usage of Meidung is definitely the same specific one to that of the Amish as you describe.
Pretty queer that Pen Dutch, and the U.S.-German. Due to the extreme isolation of the settlers, the language did not develop - much. For European speakers it's like taking a step back in time. I guess this is true of the French spoken in Canada to some extent too (anyone?).

BTW, how many Amish live over there?