Idiot

Usually we think of an idiot as someone who is very foolish or stupid.
 
In the field of mental health, the term idiot was once commonly used to describe a person with the mental ability equal or inferior to a two-year old child. However, the term idiot and other terms such as moron and imbecile are no longer used by mental health professionals.
 
Similarly, the term ‘idiot savant’ is no longer used to describe someone who, while mentally challenged, also possesses some remarkable mental ability such as memorization or rapid mathematical calculation. This condition is now known as the ‘savant syndrome’.
 
These outdated meanings are a long way from the original meanings of the word idiot.
 
Idiot can be traced back to the ancient Proto-Indo-European (PIE) roots swedyos and swe (something personal, distinct, one’s own) which are the source of Greek idio and idios, also meaning something personal and one’s own, but with the additional sense that this something was a bit peculiar or unusual. A person who possessed such idiosyncratic or even eccentric characteristics was known as an idiotes. 
 
A couple of examples from classical Greece illustrate this sense of the word idiotes. First, at that time, the most common form of oral and, later, written cultural expression was poetry. A person who for some strange reason preferred prose rather than poetry was known as an idiotes.
 
Second, the classical period of Greece is well-known as the time and place when the concept of democracy was beginning to take form; i.e., the notion that by discussion and voting each citizen can participate in making decisions which affect the life of the community. It was considered a high honor for a person to be elected by fellow citizens to hold public office in the community. (It should be noted that these citizens were men—not women, children, or slaves). Occasionally, some laymen, for personal or idiosyncratic reasons, did not participate in the democratic process. Such people were known as idiotes. 
 
In this context, an idiotes was a person with their own peculiar set of values and priorities—a person who chose to be outside the mainstream of social and cultural life.
 
Related words include idiom, idiopathic, idiosyncrasy, and idiotic.  
 
Reference: Online Etymological Dictionary, https://www.etymonline.com/
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