
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!

spinning mill
An industrial facility where yarn is spun from fibers such as cotton or wool.
Die alte Spinnerei am Fluss wurde zu modernen Wohnungen umgebaut.
The old spinning mill by the river was converted into modern apartments.
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.
madness / amazing
Wahnsinn and Spinnerei can both disparagingly describe a crazy action or idea. Wahnsinn is a strong, immediate judgment, especially of something extremely unreasonable or dangerous. Spinnerei refers more to a nonsensical idea or foolish undertaking.

Die Idee, für den Ausflug ein Schloss aus Pappe zu bauen, ist eine Spinnerei.
The idea of building a cardboard castle for the outing is nonsense.
Bei diesem Sturm ohne Ausrüstung aufzusteigen, ist Wahnsinn.
Trying to climb in this storm without any equipment is madness.