According sources in the Wikipedia article about: idiot:
The word "idiot" comes from the Greek noun ἰδιώτης idiōtēs 'a private person, individual' (as opposed to the state), 'a private citizen' (as opposed to someone with a political office), 'a common man', 'a person lacking professional skill, layman', later 'unskilled', 'ignorant', derived from the adjective ἴδιος idios 'personal' (not public, not shared). In Latin, idiota was borrowed in the meaning 'uneducated', 'ignorant', 'common', and in Late Latin came to mean 'crude, illiterate, ignorant'. In French, it kept the meaning of 'illiterate', 'ignorant', and added the meaning 'stupid' in the 13th century. In English, it added the meaning 'mentally deficient' in the 14th century.
The word "idiot" comes from the Greek noun ἰδιώτης idiōtēs 'a private person, individual' (as opposed to the state), 'a private citizen' (as opposed to someone with a political office), 'a common man', 'a person lacking professional skill, layman', later 'unskilled', 'ignorant', derived from the adjective ἴδιος idios 'personal' (not public, not shared). In Latin, idiota was borrowed in the meaning 'uneducated', 'ignorant', 'common', and in Late Latin came to mean 'crude, illiterate, ignorant'. In French, it kept the meaning of 'illiterate', 'ignorant', and added the meaning 'stupid' in the 13th century. In English, it added the meaning 'mentally deficient' in the 14th century.