Something that's idiotic is very, very stupid. It would be idiotic to turn down your lottery winnings because you're too lazy to go claim them.
An idiotic song is boring, repetitive and has ridiculous lyrics, and an idiotic TV show is badly written and terribly acted. Your dad might describe his dog as idiotic if she manages to find a skunk every single time she goes outdoors. The adjective idiotic is informal, and it comes from the Latin idioticus, "uneducated or ignorant," with the Greek root idiotikos, "unprofessional or unskilled."