I am curious as to what is the origin of the expression "nu" as "ja", as it is quite a strange divergence from the original word.
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"Nu" is a word which is used regionally to express agreement or encouragement and then only in spoken language.
My grandmother was from Sudetenland which is now Czech Republic she used it. Also friends of mine from an area South of Leipzig in Saxonia are using it.
They usually say "Nu, nu" - I almost always heard them saying it twice in a row.
"Kann ich noch eine Tasse Kaffee haben?"
"Nu, nu ..."
Then the person would pour you a cup of coffee.