
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.

obstructed
Hindered or blocked in an action or movement. This is the past participle form of the verb 'behindern' (to hinder).
Der Verkehr wurde durch einen umgestürzten Baum behindert.
Traffic was obstructed by a fallen tree.

retarded
(Colloquial, derogatory) Used as an insult to express that one finds something or someone stupid or nonsensical.
Diese neue Regelung ist doch total behindert!
This new rule is totally retarded!
'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 is a general negative judgment of something unwise or nonsensical. In its colloquial, derogatory use, behindert is used as an insult and does not neutrally describe the action; instead, it insults a person or thing. Dumm is appropriate for an objective criticism; behindert should be avoided in this sense.

Mit „Das ist behindert“ benutzte er ein diskriminierendes Schimpfwort.
By saying “Das ist behindert,” he used a discriminatory slur.
Der Vorschlag war dumm, weil er das eigentliche Problem nicht löste.
The suggestion was stupid because it did not solve the actual problem.