Funnily enough, it was introduced as a euphemism to "idiot" or "cretin", primarily in a medical context, because those other words were considered too unsavoury

Funnily enough, it was introduced as a euphemism to "idiot" or "cretin", primarily in a medical context, because those other words were considered too unsavoury. Now, I daresay "retard" is considered worse than "idiot".