1: a foolish or stupid person

Idiot Has Greek Roots

The Greek adjective idios means “one’s own” or “private.” The derivative noun idiōtēs means “private person.” A Greek idiōtēs was a person who was not in the public eye, who held no public office. From this came the sense “common man,” and later “ignorant person”—a natural extension, for the common people of ancient Greece were not, in general, particularly learned. The English idiot originally meant “ignorant person,” but the more usual reference now is to a person who lacks basic intelligence or common sense rather than education.

Source: https://www.merriam-webster.com/dictionary/idiot