idiotisch
idiotic
Extremely stupid, senseless, or thoughtless. It is a strongly derogatory, colloquial expression.
Es war eine idiotische Idee, bei diesem Sturm schwimmen zu gehen.
It was an idiotic idea to go swimming in this storm.

idiotic
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idiotic
Extremely stupid, senseless, or thoughtless. It is a strongly derogatory, colloquial expression.
Es war eine idiotische Idee, bei diesem Sturm schwimmen zu gehen.
It was an idiotic idea to go swimming in this storm.

This word is very strongly judgmental and is perceived as insulting. It belongs to colloquial language and should definitely be avoided in formal or polite conversations.
Sein Verhalten auf der Party war einfach nur idiotisch.
His behavior at the party was simply idiotic.
The adjective 'idiotisch' (idiotic) is derived directly from the noun 'der Idiot' (idiot). Both words have a strongly negative and insulting meaning.
Nenn mich nicht Idiot, das ist verletzend.
Don't call me an idiot; that's hurtful.
'Idiotisch' (idiotic) is often used to judge actions, ideas, or statements as extremely senseless. Typical combinations are 'eine idiotische Idee' (an idiotic idea), 'eine idiotische Frage' (an idiotic question), or 'idiotisches Verhalten' (idiotic behavior).
Das war eine idiotische Frage, natürlich funktioniert das nicht.
That was an idiotic question; of course that doesn't work.
stupid / unfortunate
'dumm' and 'idiotisch' both describe something as not clever or as senseless. 'dumm' is the more general word and can be meant more mildly or more strongly. 'idiotisch' is much stronger and very derogatory. It is especially appropriate when you are annoyed about an idea, a rule, or a behavior and find it extremely unreasonable. When used for people, 'idiotisch' can sound very insulting. If you want to stay more neutral, 'dumm' is usually better.

Diese idiotische Regel macht die Arbeit nur komplizierter.
This idiotic rule only makes the work more complicated.
Mein Nachbar hat eine dumme Frage gestellt, aber niemand hat gelacht.
My neighbor asked a stupid question, but nobody laughed.