An individual who can not coherently participate in and contribute to society. An idiot is considered 'less', and not respected by his peers. One may use the term 'idiot' when referring to a classmate or coworker when they make mistakes repeatedly, or say the most ridiculous of things.
'I can't believe that Zach said that. He is such an idiot.'
'What makes Zach think that he can just do that ? What an idiot.'