What is the origin of the idiom "have rocks in one's head" and when to use it?

The idiom "have rocks in one's head" likely originated from the concept that having rocks or stones in one's head would obstruct clear thinking and intelligence. Over time, it evolved into a metaphorical expression to describe someone who is perceived as foolish or lacking mental acuity. It is typically used in a lighthearted or humorous manner to convey a playful criticism of someone's mental acuity or decision-making abilities.