An idiot is someone who does foolish things. If you take your little brother's dinosaur lunchbox to school instead of your own brown-bag lunch, you might feel like an idiot. But don't be too hard on yourself — we all make mistakes.
The word idiot originated in the 1300s, from the Old French word idiote, which meant "person so mentally deficient as to be incapable of ordinary reasoning." Today, though, idiot describes anyone who does foolish things, especially things that inconvenience others. If you put the Thanksgiving turkey in the oven but forget to turn it on, in four hours, you'll have a cold turkey and a bunch of relatives calling you an idiot.