Other forms: dimwits
A dimwit is a person who's not very smart. If you catch your friend sending a text message while he's driving, you can yell at him to put his phone down and call him a dimwit.
No one wants to be called a dimwit, since it implies incompetence and a low IQ — it's important to use this word carefully to avoid offending someone. Still, when your brother leaves a pizza unattended in a room with his two big dogs, you'll probably have to say, "Well of course they ate it, you dimwit." The word was coined in the US, as 1920s college slang.