unreasonable
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unreasonable
Not guided by reason or good judgment; irrational or reckless.
Es ist unvernünftig, bei diesem Gewitter eine Bergwanderung zu machen.
It is unreasonable to go hiking in the mountains during this thunderstorm.
'Unvernünftig' (unreasonable) is the direct opposite of 'vernünftig' (reasonable). It is formed with the prefix 'un-', which expresses negation. The base is 'die Vernunft' (reason).
Sei doch nicht so unvernünftig und fahr bei diesem Nebel langsam!
Don't be so unreasonable; drive slowly in this fog!
You distinguish the meanings by the noun being described. If 'unvernünftig' (unreasonable) refers to a person or their behavior, it means 'reckless'. If it refers to a price or a demand, it means 'excessively high'.
Es war eine unvernünftige Entscheidung, den Job zu kündigen.
It was an unreasonable decision to quit the job.
In the meaning of 'irrational', the word is often used as an appeal or criticism to bring someone to their senses. It usually describes actions, decisions, or people.
Deine Idee ist völlig unvernünftig, das kann nicht funktionieren.
Your idea is completely unreasonable; that cannot work.
sensible
'sinnvoll' and 'unvernünftig' form an opposite pair when talking about decisions or behavior. 'sinnvoll' means that something has a good purpose and seems sensible. 'unvernünftig' says that someone is acting without thinking carefully or is not taking a risk seriously. The difference is in the perspective: 'sinnvoll' mainly evaluates the usefulness of an action, while 'unvernünftig' focuses more on the poor or careless judgment behind it.
Elif fand es unvernünftig, ohne Frühstück den ganzen Tag zu arbeiten.
Elif thought it was unreasonable to work all day without breakfast.
Es ist sinnvoll, dass das Team vor dem Projekt klare Aufgaben verteilt.
It is sensible for the team to assign clear tasks before the project.