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.
"STOP, Hawaii 5-O!!!!"
....blah, blah, blah- small struggles, throw a few punches - handcuffs criminal and stuffs them in the back of a patrol car ....
"Book 'em Danno", said Steve Garrett.
On a side note, the division was actually called "Hawaii Five-0" (as in Zero, no the letter "o") but IMDB has the show listed with the letter "o" in the tittle.