der Wahnsinn
amazing / unbelievable
A colloquial expression for something unbelievable, crazy, or impressive. Often used as an exclamation.
Du hast die Prüfung bestanden? Das ist ja der Wahnsinn!
You passed the exam? That's absolutely amazing!

madness / amazing
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amazing / unbelievable
A colloquial expression for something unbelievable, crazy, or impressive. Often used as an exclamation.
Du hast die Prüfung bestanden? Das ist ja der Wahnsinn!
You passed the exam? That's absolutely amazing!

The word 'Wahnsinn' (madness/insanity) is made up of 'Wahn' (a false idea or delusion) and 'Sinn' (reason, mind). So it originally describes a state in which the mind is clouded.
Seine Eifersucht trieb ihn an den Rand des Wahnsinns.
His jealousy drove him to the edge of madness.
In colloquial speech, 'Wahnsinn!' or 'Der Wahnsinn!' is often used as a short exclamation to express strong amazement, enthusiasm, or even horror.
Du hast im Lotto gewonnen? Wahnsinn!
You won the lottery? Amazing!
The meaning 'unbelievable' or 'impressive' is strongly colloquial. In formal or written contexts, the word is used less often in this way.
Das Konzert war der absolute Wahnsinn, die Stimmung war unglaublich.
The concert was absolutely amazing; the atmosphere was incredible.
To describe an unreasonable action, the structure 'Es ist Wahnsinn, etwas zu tun' (It is madness to do something) is often used. The word 'reinste' (pure) can intensify the negative meaning.
Es ist der reinste Wahnsinn, ohne Licht mit dem Fahrrad zu fahren.
It's pure madness to ride a bicycle without lights.
The original meaning 'mental illness' is used today mainly in literary, historical, or very elevated contexts. In everyday life, people talk about mental illness differently.
Der Künstler litt unter Verfolgungswahn und war dem Wahnsinn nahe.
The artist suffered from paranoia and was close to insanity.
mind / reason
'der Verstand' and 'der Wahnsinn' are opposites when it comes to clear versus unreasonable thinking. 'der Verstand' means the ability to think logically and judge reasonably. 'der Wahnsinn' can describe an extremely unreasonable or dangerous action; in older or literary language, it can also mean severe mental confusion. In everyday speech, people say 'Das ist Wahnsinn' (That's madness) when something seems completely excessive or unreasonable. 'Verstand' fits when the idea is calmness, logic, and control.

Bei diesem Sturm ohne Jacke auf den Berg zu gehen, ist reiner Wahnsinn.
Going up the mountain without a jacket in this storm is pure madness.
Trotz der Angst nutzte sie ihren Verstand und rief zuerst die Polizei.
Despite her fear, she used her reason and called the police first.