Other forms: idiocies
Idiocy is extreme stupidity. If you skied down a dangerous slope with little experience and no helmet, your friends would be right to comment on your idiocy.
When someone behaves in a foolish way, showing no evidence that they've thought about their actions, you can call it idiocy. You could scold your little brother, "Your idiocy in the bouncy house made it impossible for anyone else to have fun at the party," or criticize the idiocy of your governor's decision to refuse federal funds for school lunches. The noun idiot came before idiocy, and both words originally had psychiatric meanings that are now obsolete.