The noun idiot means a person of lower-than-normal intelligence or someone who does foolish things, like your neighbor who car got stolen because he left it unlocked with the key in the ignition. What an idiot.
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 "idiot."