Was für ein Mumpitz!
I'm not sure why you need to use "ein" here, as "Mumpitz" is a masculin noun and the preposition "für" should be followed by the accusative form "einen".
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Sign up to join this communityYou are right "Mumpitz" is masculin. But you misinterpret the structure. It's not that something is done for the Mumpitz, or something similar.
Was für ein Mumpitz
is short for
Was ist das für ein Mumpitz
so here "was für ein" is synonmym to "welche Art", "welch ein" and that's why Mumpitz is in nominative case here.