Have you noticed how when you state a fact or answer a question, the person who didn’t know, calls you the retard?
“Well I didn’t know that, did I? Retard!”
It has got to be some kind of mental defense mechanism to make themselves feel better at your expense.
Also, when uttered in a social setting, it is usually followed by hollow laughter from the other people present, as though they are in on the joke or how retarded you are for knowing something they didn’t.