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Quiet retard.

or Quiet, retard.

They should be implying for the person you are talking to to be quiet and also call them a retard. Thanks
Comments  
my opinion of view is the second one is rt,
In the first sentence
quiet retard.

quiet is an adjective to the noun retard.

as you could say:
noisy retard
beautiful retard

in the second sentence:
Quiet, retard.

quiet is a verb.

like:
Move on, retard.

So, the second one fits into your explanation.
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I thought " retard" was a humilliating word. Please, could we have some examples.
yes, it is.

dictionary.cambridge.org:

retard
noun [C] OFFENSIVE
a stupid or mentally slow person:

I'm not playing with him, he's a total retard.