The English word “idiot” actually comes straight from the ancient Greek word “ιδιώτης”.
Etymology (idiot / ιδιώτης):
“Ιδιώτης” was a man who would only deal with his personal affairs and not the political affairs of his country.
But, how does this relate to the meaning of the word “idiot” in English, (where idiot = stupid)?
Within the frames of the ancient Greek democracy, the citizens where expected to be very energetic in the governing of their own city state. Otherwise, they were considered inadequate and completely incompetent for their country. So the word "ιδιώτης" in ancient Greek had a degrading, even insulting meaning. This negative meaning is very close to the meaning of the word “idiot” in English.
In modern Greek “ιδιώτης” means individual, a private citizen, a person without a public profile. There is no negative meaning at all.
Όταν μιλάει ο Πρωθυπουργός, εκπροσωπεί τη χώρα. Δεν έχει το δικαίωμα να εκφράζει προσωπικές απόψεις, όπως ένας ιδιώτης.
When the Prime Minister speaks, he represents the country. He has no right to express personal opinions, as if he was just an individual.