Its a combination of "Piss" and "Nissan"
A term to make fun of Nissan cars. Especially when in a bad condition or quite old.
This word is also used in other languages, like German, with the same meaning. (In Germany Nissan cars have a very bad image.)
Also occurs in the PC-game "Painkiller", since the developers didn't have the permission to add Nissan Cars to the game, so the batch was changed to "Pissan".
Look at this guy! He still drives his fucked up Pissan!
What car does he drive? I don't know, some kind of Pissan or something.