Here's what we know:
* It's likely a folk idiom: This means it emerged organically from everyday language, rather than being coined by a specific person or event.
* It's a simile: It compares a person's intelligence (or lack thereof) to the perceived lack of intelligence in a box of rocks.
* The earliest recorded use: The earliest known instance of the phrase in print appears in the 1960s. However, it was likely in use orally for many years before that.
Why is it effective?
The idiom works because:
* It's vivid: The image of a box of rocks conjures up a clear picture of something that is utterly inanimate and devoid of thought.
* It's relatable: Most people have seen a box of rocks, and the idea of them being "dumber" than something is inherently funny.
* It's exaggerated: The hyperbole emphasizes the point and makes it more memorable.
While the exact origin remains unclear, the phrase "dumber than a box of rocks" is an enduring testament to the power of simple, relatable language.