The origin of this idiom is unclear. However, it's likely rooted in the idea that a hard head is associated with stupidity. This association could come from several sources:
* Physical injury: People with head injuries can exhibit changes in behavior and cognitive abilities.
* Lack of mental flexibility: The phrase "hard-headed" can mean stubborn or inflexible, which can be seen as a negative trait.
* Superstitions: In some cultures, rocks or stones were associated with evil spirits or bad luck.
The idiom "have rocks in your head" likely evolved from these associations, becoming a way to describe someone who is unintelligent, stubborn, or prone to making poor decisions.
Here's a breakdown of its use:
* Negative connotation: It's always used to express disapproval of someone's actions or thinking.
* Informal usage: This idiom is considered informal and is often used in casual conversations.
* Alternative expressions: There are similar idioms with similar meanings, like "have sand in your craw" or "be a few bricks short of a full load."
It's important to note that this idiom can be offensive, especially if used in a discriminatory or hurtful way.