'idiocy' originates from Greek, specifically the word 'idiōtēs', where 'idios' meant 'private' or 'one's own' and referred to a lay or unskilled person.

'idiocy' came into English via Latin and Old French forms (Latin 'idiota', Old French 'idiotie') and Middle English 'idiocie', eventually becoming the modern English word 'idiocy'.

Initially, related words referred to a private or unlearned person ('one who is not a specialist'); over time the sense shifted toward 'lack of intelligence' and then to the modern senses of 'extreme foolishness' or, in older medical/legal usage, 'severe intellectual disability'.