A person (usually a male) that appears magically in the backseat of your car while you're driving. They aren't very popular today, but were very popular back in the 50's. Very few these days have encountered a Backseatsman, but those who have will never forget their experience.
Driver: Hey! How did you get in here?!
Backseatsman: I don't really know how i got in here... but here i am. And here we are. :D