A person who's not so smart might be called a dunderhead. If you manage to start a fire while attempting to fry an egg, your mom might call you a dunderhead.
Dunderhead is a more colorful way to say "dummy" or "stupid." It's derogatory, but not the meanest word for someone making a really foolish move: "You dunderhead!" tends to sound a little bit amused. You might call a goofy kid a dunderhead, or call yourself a dunderhead when you make the same silly mistake over and over again. The origin isn't certain, though the dunder part may come from the Middle Dutch word donder, or "thunder."