Most of our insults against intelligence come from medical terminology originally. Every word eventually starts being used offensively, and then a new one gets coined and the cycle continues.
I assume you're being sarcastic, but moron, retard, stupid, dumb, idiot, mad, imbecile, mongoloid, spastic, lame, lunatic, maniac and psycho were all used as medical terms before they became stigmatic. Filter out sexual and scatological insults and there is very little left.