The name of Ellis' high school truck club, as seen on his shirt in the video game Left4Dead 2. It is a play on words; "shifters" referring to trucks/cars, and if you haven't already figured it out, the whole word sounds like "bull shitters".
I sabotaged the Bullshifters meeting with donkey poo.
The act of shifting a manual car to their highest gear and leaving it there regardless of how long it takes for the car to accelerate. The only time a bullshifter would shift down would be at the risk of stalling. Bullshifting is primarily a result of laziness.
Passenger: "Shouldn't you shift down? You're going freaking slow."
Driver: "Nah. I'm bullshifting."