(colloq.) Similar to a 'third wheel'. Not to be confused with 'back seat driver'.
From the situation found sometimes in cars with front bench seats where you are in the driver's seat, your sweetie is in the passenger seat, and some other individual is sitting between you in the middle.
This person is always either incredibly uncomfortable at being 'in the way', or completely oblivious to the phenomenon, and incredibly enthused about something to the point of being irritatingly happy and bouncy.
Sasha Gabba Hey! advises that a far more common form of 'MSP' is when you have 3 guys in the front cab of a truck/ute. The guy in the middle is usually the youngest/the apprentice. This means the others have a much easier time ashing their cigarettes out the window, and have more 'stuff to hold onto' when going around corners. SGH! refers to this as the 'heirarchy of despatch'.