Why idiot, imbecille and moron are okay but retard is off-limits? All of these words have been used in a psychological classification system in the past yet retard seems to be the only one people take issue with.
Idiots. —Those so defective that the mental development never exceeds that or a normal child of about two years.
Imbeciles. —Those whose development is higher than that of an idiot, but whose intelligence does not exceed that of a normal child of about seven years.
Morons. —Those whose mental development is above that of an imbecile, but does not exceed that of a normal child of about twelve years.
Edmund Burke Huey, Backward and Feeble-Minded Children, 1912