To accuse another of having weak kidneys, lungs or heart is not a crime; on the other hand, saying someone has a weak brain IS a crime.
To be considered stupid and to be told so is far more painful than being called gluttonous, mendacious, violent, lascivious, lazy or cowardly. Every weakness, every vice has found its defenders, its rhetoric, its ennoblement, its exaltation . . . but stupidity hasn't.