backseat dm 

The "backseat dm" or "backseat dungeon master" brings a thoroughly marked and noted rulebook, and is quick to jump in whenever a rule is broken or ignored.
The nerd/D&D version of backseat driving

Player 1: That's not really how it's done, but I'll allow it.
Player 2: Wait a minute! That's against rule 534 on page *flips open rulebook*
P1: I'M the DM here. Don't backseat dm me.

backseat dm by shieldswinger May 15, 2018

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!"

backseatsman by Charliiiiie May 30, 2006

Backseatsman 

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

Backseatsman by Autumn-Hope August 21, 2010

Backseatsman 

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...

Backseatsman by CapnNorric July 13, 2010