A knucklehead is a stupid person, but you often say it when you are not really angry. You might say it to a friend who did something silly, with a half-smile, the way you say you idiot without wanting a fight. It is informal American English, a little old and comic. Parents and coaches in American films use it. It is much softer than knuckle-dragger. Nobody uses it in careful writing. The word puts knuckle and head together, as if the head were thick as a knuckle — though actually, people who say it are rarely thinking about bones. They just mean silly.