die Spinnerei
nonsense
A crazy, nonsensical idea or a foolish undertaking. Used colloquially and disparagingly.
Ein Haus auf dem Mars zu bauen? Das ist doch eine totale Spinnerei!
Building a house on Mars? That's complete nonsense!

nonsense / spinning mill
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nonsense
A crazy, nonsensical idea or a foolish undertaking. Used colloquially and disparagingly.
Ein Haus auf dem Mars zu bauen? Das ist doch eine totale Spinnerei!
Building a house on Mars? That's complete nonsense!

Both meanings of 'Spinnerei' derive from the verb 'spinnen'. 'Spinnen' literally means 'to make yarn', but colloquially it also means 'to be crazy' or 'to talk nonsense'.
Er spinnt doch, wenn er glaubt, das funktioniert.
He must be crazy if he thinks that will work.
The meaning 'crazy idea' is colloquial and is often used disparagingly to dismiss a plan or statement as unrealistic or stupid.
Seine Idee, ohne Geld eine Weltreise zu machen, ist doch eine reine Spinnerei.
His idea of traveling around the world without money is just pure nonsense.
In the colloquial meaning, the word is often intensified with adjectives such as 'total' (complete), 'rein' (pure), or 'glatt' (sheer) to emphasize the absurdity of an idea.
Glaub diese Geschichte nicht, das ist eine totale Spinnerei.
Don't believe this story; it's complete nonsense.
You can recognize the meaning 'factory' from the context. Words like 'Gebäude' (building), 'Industrie' (industry), 'alt' (old), 'renovieren' (to renovate), or location information clearly point to the industrial facility.
Die alte Spinnerei wurde zu einem Kulturzentrum umgebaut.
The old spinning mill was converted into a cultural center.
nonsense
'Quatsch' and 'Spinnerei' can both refer to something nonsensical. 'Quatsch' is more general and can be a false statement, a silly idea, or any kind of nonsense. 'Spinnerei' is used more for a crazy idea or a plan that seems unrealistic. So it is often about something someone imagines or intends to do. If you only want to say that something is not true or is stupid, 'Quatsch' fits. If an idea sounds exaggerated, strange, or hardly feasible, 'Spinnerei' fits better.

Heute hält sie den Plan, in zwei Wochen ein Haus zu kaufen, für reine Spinnerei.
Today she considers the plan to buy a house in two weeks pure nonsense.
In der E-Mail steht viel Quatsch über das neue Projekt.
The email contains a lot of nonsense about the new project.