numbskull
nounUK /ˈnʌmskʌl/
US /ˈnʌmskʌl/
A numbskull is a silly person who does not think very well.
He forgot his own birthday, the numbskull!
OTHER MEANINGS
2. A person who acts foolishly in a careless way.
Don't be a numbskull and leave your keys in the door.
3. A playful insult for a clumsy or forgetful friend.
You numbskull, you dropped the whole tray!
4. Someone who lacks good judgment in a funny, harmless way.
That numbskull tried to dry his cat with a hairdryer.