Lunkhead
/ˈlʌŋkhed/
Definition 1 of 1
Pro Tip 1/3Informal Insult
Used among friends or peers in a light-hearted way; not severe but can be disrespectful. 
Stop spacing out and focus, you lunkhead!
Pro Tip 2/3Avoid at Work
Inappropriate in professional settings; using 'lunkhead' at work could be rude and offensive. 
I managed to bite my tongue before calling my boss a lunkhead.
Pro Tip 3/3Cultural Nuance
The term is mostly used in American English and may not be understood or taken lightly elsewhere. 
When I called him a lunkhead in London, they didn't get the joke.