[tharkun__]

I only knew it from Hungarian, which I guess is a slavic enough language to compare: https://en.wikipedia.org/wiki/N%C3%A9meth:

In Hungarian, német means "German" (the word has Slavic origin, literally meaning "he does not speak", since German is not a Slavic language)

[kingofpandora]

Hungarian isn't Slavic.

[petre]

The word is probably borrowed into Hungarian from Slavic. We have a similar word in our language as well, but it's used as someone who doesn't understand. Also used interchangeably with Turk.

[anticensor]

Do you mean Barbar?

[petre]

No, it's very similar to nemec, basically the same thing also borrowed from Slavic. The "c" ending is pronounced as "ts". The full phrase goes like: "Why don't you understand, are you German/Turk?" (as in not understanding the language). There's also a place, a citadel and river with that name, which your ancestors might have been familiar with:

https://en.wikipedia.org/wiki/Siege_of_Neam%C8%9B_Citadel

Barbar means what it means in Greek, because that's where it's borrowed from.