a patrol car.
Orgin: late 80's Cali.
back then all police cars at one time where Mustangs, 5.0 engine model, on the front and side of the car it read in chrome letters the engine model. 5.0.
it refers to the car they patrol in.
now it also refers to the police officer himself.
"5-O keeps making rounds through my neighborhood like we are causing problems".