Verb. The act of having a very important, funny, or rediculous idea but minutes later vanishes (as violently as a sneeze).
Dustin: "So what where you going to say at lunch?"
Dillon: " I cant remeber i had a brain sneeze during fourth hour!"
"As I had the perfect idea for senior prank, a brain sneeze caught me off guard."
When your brain needs to sneeze, but it can't because it's a brain, so it just hurts.
This snowcone is giving me a brain sneeze.