Idiot derived from the Ancient Greek ἰδιώτης (a private citizen). In Athenian Democracy a free man had to participate to public affairs. Therefore, ιδιώτης was the person who opposed to public affairs and this was very deprecatory for a free Athenian citizen. That's why the term has negative meaning in English. On the other hand, in Modern Greek ιδιώτης is someone who operates privately and it is not an insult or something else.
Private person and idiot became synonyms in the only real democracy. This is the story: In ancient Greek if such an incredible thing happened that a person selected by lottery to take care of public affairs said "no my private affairs prevent me from taking care of more", then the others started to point finger on him wherever he went, saying "look there goes the ιδιώτης (private man=idiot) who only can look after his own business.
Apart from being used as the wellknown insult, idiot was a medical term used in the past in both English and German, but has been completely replaced by other more scientific/neutral terms. It is interesting to see it crop up again in Greek psychiatric terminology. It would be interesting to know if it is acutally still being used by Greek psychiatrists today, or if it has been replaced as well.
But it's not acceptable nowadays, except perhaps in the term 'village idiot', though is that acceptable? to use idiot to mean low IQ. How do you say 'challenged' in Greek? My next door neighbour is totally deaf, and everything that results from that (ie speech), but he certainly isn't an idiot. I rely on him for all my streetwise, or rather fieldwise, expertise.
For fun, I can add to these comments by bringing up the medical term "idiopathic" which is "not" related to psychiatry.
Idiopathic: relating to or denoting any disease or condition that arises spontaneously or for which the cause is unknown.
So the pathology is not yet known in the public domain? It is private?