Dunderhead
/ˈdʌndərhed/
Definition 1 of 1
Pro Tip 1/3Informal Use
Best used among friends or in casual conversations, not in professional or formal settings. 
After losing the game, Jim jokingly called himself a dunderhead.
Pro Tip 2/3Humorous Insult
Often used humorously rather than to seriously insult someone. 
You locked your keys in the car? You dunderhead!
Pro Tip 3/3Mild Offense
It's a mild term of offense; not as harsh as other insults but can still hurt if not used jokingly. 
Lisa smiled when her brother called her a dunderhead, knowing he didn't mean it.