[[Maas]]
Do you have a source to prove that German Maas is borrowed from Dutch? It doesn't make much sense to me because speakers of Central German and German Low Franconian have always lived so close to the river that the name can impossibly have been lost at some point in time. Maas would be the expected outcome of Germanic *Masō in these areas (which all have apocope). This is not to say that word “Maas” in modern standard German isn't based on Dutch at least to some degree. But I'm not sure about your statement that the word had died out in all languages but Dutch. See also the Rheinisches Wörterbuch where dozens of sayings involving the Maas are given: [1].
I think it would be more convincing if attestations could be found in OHG and MHG. Those would provide the "missing link" between Proto-Germanic and modern German. Dutch is the only descendant currently listed that has attestations going back that far.
Well. You wouldn't find Old High German texts from these areas probably. MHG maybe. But it's you who made the claim that the word died out in all languages but Dutch. Now I don't see how the word can possibly have died out in the dialect of Aachen. That's a pretty daring claim when the river is less than 20 km from this city. So my opinion is that common sense proves that the Aachen dialect has always had a word for a major river less than 20 km from itself. And since (in my definition) it is a dialect of German, the word hasn't died out in German.
But then where are the attestations? Did people not write anything in Aachen until a few centuries ago?
Yeah, you're right of course. One should look for attestations, for example the earliest attestation in a High German text. That has probably been done and I should check it sometime... But I still don't get on what you base your claim. If you say that a word died out in all Germanic languages except one, then I would presuppose that you yourself should have checked these languages for their earliest attestations. You wrote that the German word for "Maas" had died out (or never existed, I don't know). You also wrote that modern German "Maas" is borrowed from Dutch. Now if you say that you need proof, I guess. It's not me who needs to prove the contrary. And of course, if an unfounded claim makes perfect sense then I don't bother at all. (I'm not someone who says we need to quote seven books about something that is common sense.) But this is one thing that just doesn't make much sense to me (personally).-- Best regards, I'm going to bed.