idiotic

1. Showing foolishness or stupidity.
2. Exhibiting idiocy.

adjective

So senseless as to be laughable: absurd, foolish, harebrained, imbecilic, insane, lunatic, mad, moronic, nonsensical, preposterous, silly, softheaded, tomfool, unearthly, zany. Informal cockeyed, crazy, loony, loopy. Slang balmy², dippy, dopey, jerky, sappy, wacky. See ability/inability, knowledge/ignorance.

Definition: very stupid
Antonyms: brainy, genius, intelligent, sensible, smart

The adjective has one meaning:

Meaning #1: insanely irresponsible
  Synonym: crackbrained