bonehead

This term is highly informal and carries a strong derogatory tone, typically used to describe someone who lacks common sense or makes obvious, avoidable mistakes. It suggests a certain stubbornness in one's stupidity, implying that the person's head is as dense as a bone.

Because of its casual and insulting nature, it is unsuitable for professional or formal environments. It is more aggressive than "silly" but less profane than many other insults, often used in sports or casual arguments to criticize a specific blunder.