idiot is a term used to describe someone who is extremely foolish or stupid.
In medical history, idiot was once used to refer to people with very low IQ (below 25).
The word "idiot" comes from the Greek word "idiotes" meaning "private person" or "layman".
Modern usage examples:
1. "Don't be such an idiot!"
2. "I felt like a complete idiot when I forgot my own phone number."
3. "That was an idiotic thing to do."