Comment: Which legal argument

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Which legal argument

does calling someone retarded fall under?

By a social standpoint, we call that an ad hominem. If you don't know, an ad hominem is an attack on one's character to avoid a logical discussion of the facts being discussed in the debate.

A lawyer (or one training to be one) would not normally resort to such underhanded and irrational techniques as it provides no sway to one side of the argument or another. You, my friend, are no lawyer.