The Viennese (Austrians?) use “Na geh'” (a distant cousin to the English expression “Now get along with you”) to express general disbelief and mild protest–sometimes both at one time. It is almost exclusively a genial expression.
A Viennese friend of mine told me a joke that beautifully illustrates the use of the phrase:
Two animals meet for the first time.
The first one says, “I’m a Wolfshund. My father was a wolf and my mother was a dog. What are you?”
The second one says, “I’m an Ameisenbär.” [Ameisenbär is the German word for anteater and translates literally as “ant bear”.]
The Wolfshund responds, “Na geh’“.
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