An idiot is someone who acts foolishly or makes very poor decisions, showing a lack of good judgment or common sense. If your friend tries to ride their bike down a steep hill with no brakes, you might think, “Don't be an idiot!” You're warning them that their choice could lead to serious consequences.

The word carries real sting when you use it. Calling someone an idiot tells them you think they're being remarkably foolish or careless. It's harsher than saying someone made a mistake, because it suggests the person should have known better. When you call yourself an idiot after forgetting your homework for the third time, you're expressing frustration at your own carelessness.

People sometimes use idiot too casually, tossing it around when they're annoyed. But the word works best when someone's behavior is genuinely reckless or thoughtless, not just different from what you would do. There's a big difference between calling someone an idiot for trying a food you don't like, and using the word when someone ignores obvious danger.

In older times, idiot had a more specific medical meaning that's no longer appropriate to use. Today it's simply a blunt, somewhat harsh insult for foolish behavior.