It's similar to saying someone is "thick" or "airheaded". However, "dimsided" is often used in a more lighthearted way, suggesting that the person is simply not very bright, rather than being intentionally foolish.
Here's an example:
* "My brother is so dimsided, he once tried to start the car with the key in the ignition."
The word "dimsided" is not commonly used in formal writing or speech. It's more likely to be used in casual conversation with friends or family.