Someone who's dim-witted isn't very smart. It's not a very nice way to describe someone, so you might want to keep your thoughts about your dim-witted cousin to yourself.
The adjective dim-witted comes from the 1920s U.S. college slang term dimwit, which combined dim, "low intensity," with the sense of wit meaning "intelligence." It would be dim-witted to jump off the roof with an open umbrella to see if you'll float gently to the ground. (Spoiler: You won't.)