behindert
disabled
Having a physical, mental, or psychological limitation or disability.
Für behinderte Menschen gibt es spezielle Parkplätze.
There are special parking spaces for disabled people.

disabled / obstructed
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disabled
Having a physical, mental, or psychological limitation or disability.
Für behinderte Menschen gibt es spezielle Parkplätze.
There are special parking spaces for disabled people.

'Behindert' (disabled/obstructed) is the past participle of the verb 'behindern' (to hinder, to hold up). This origin explains the meanings: a person can be 'behindert' (disabled/limited) in their abilities, and traffic can be 'behindert' (obstructed/blocked) by an obstacle.
Ein Unfall behinderte den Verkehr auf der Autobahn für mehrere Stunden.
An accident obstructed traffic on the highway for several hours.
Context tells you whether a person or a thing is meant. If 'behindert' (disabled/obstructed) refers to a person, it describes a lasting limitation. If it refers to a thing like traffic or visibility, it describes a temporary blockage.
Die Sicht war durch den dichten Nebel stark behindert.
Visibility was severely obstructed by the thick fog.
The colloquial use as an insult is strongly derogatory and discriminatory. You should know this meaning in order to understand it, but you should under no circumstances use it yourself.
Diese abfällige Bemerkung war wirklich unangebracht.
That derogatory remark was really inappropriate.
In official usage, 'behindert' (disabled) is part of many fixed terms. Examples are 'behindertengerecht' (accessible/barrier-free), the 'Behindertenparkplatz' (disabled parking space), or the 'Behindertenausweis' (disability ID card).
Das neue Museum ist vollständig behindertengerecht gebaut.
The new museum is built to be fully accessible.
stupid / unfortunate
'dumm' and 'behindert' can be used in a similar way in insulting colloquial language when someone calls something or someone nonsensical or not smart. The important difference is: 'dumm' is already negative, but still a normal word for a lack of intelligence. 'behindert' actually has to do with a physical, mental, or psychological disability. As an insult, it is very hurtful and discriminatory. For that reason, you should not use 'behindert' when you only mean 'dumm' (stupid) or 'unsinnig' (nonsensical).

Manche sagen leider „behindert“ als Schimpfwort, aber das ist verletzend.
Unfortunately, some people say 'behindert' as an insult, but that is hurtful.
Ihr habt eine dumme Entscheidung getroffen, aber ihr könnt sie noch ändern.
You made a stupid decision, but you can still change it.