A creature who thrives in backseats of things, especially cars. He can get along just fine on the back of a bike, though.
A creature made up by The Lonely Island
Driver: There's something living in the backseat of my car!!
Passenger: It's just a backseatsman...
1. Queer old gentlemen from the 1950s who sit in your backseat for a living. They will certainly die if they are separated from the backseat. If you work for The Car Service and a backseatsman appears, you can never pick up anyone else again (unless he is asleep or has Downs Syndrome). They are also lacking severely in yo-yo skills.
2. See also: The Lonely Island.
1. "You work for The Car Service! There'll always be another fare!" exclaimed the backseatsman.
2. "I'm a BACKSEATSMAN! This is what I do!"
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