Idiotic means extremely foolish or showing a complete lack of good judgment. When something is idiotic, it's so obviously wrong or senseless that it makes people wonder how anyone could have thought it was a good idea.

If someone tries to dry their wet phone in the microwave, that's an idiotic choice because microwaves destroy electronics. If a student copies answers directly from the internet without reading them and turns in a report about the wrong subject entirely, that's idiotic behavior. The word suggests actions so unreasonable that they seem almost impossible to explain.

Idiotic is a harsh word. While you might call your own decisions idiotic when you realize you've done something really dumb (“It was idiotic of me to leave my backpack on the bus”), calling someone idiotic is an insult. It's usually used about actions or ideas rather than people. You might say “that was an idiotic plan” about a scheme to sneak cookies by hiding them in your socks.

Something idiotic lacks even basic common sense.