When we say that the word idiot is derived from the Greek idiotes, it must be understood that the Greek term had no such meaning as we give to idiot, not remotely.
Its stem, idio, is the same as that which gives us “idiom,” one’s own personal or individual language, and “idiosyncrasy,” one’s own personal or individual characteristics.
So the Greek word originally meant nothing worse than a private person, an individual, a person occupied in his own affairs as distinct from one holding public office.
It acquired an extended meaning from this, one who lacked professional knowledge, whether of politics or other subjects; a layman, but in Athens there was no reflection upon one’s mentality when referred to as an idiot.
When the Romans borrowed the term, giving it the form idiota, they gave it a slight stigma, however. In their view, a man who failed to take enough interest in the affairs of state as not to hold any public office, must be one who lacked the brains for the job.
They held that contact with public life was indispensable for the full development of the intellect, and thought that none but a weak-minded person would refuse an opportunity for such service. So, to the Romans, an idiot was a person who, through lack of mental ability, was unfit for public office; hence, an ignoramus. Early English writers were somewhat confused as to the meaning when idiot was introduced into our language.
Some used it in its Greek sense and some gave it the Latin meaning, but others went further than the Romans had gone and gave it just the meaning that we give it today, a person so deficient in mental powers as to be incapable of reasoning or of self-protection.