5-0 became a slang term for law enforcement (aka police) after the hit show "Hawaii Five-O". The show was centered around a fictional division inside the State-Police of Hawaii and carried the same name as the television show ("Hawaii Five-0". The division's name gets its origin from honoring the fact that Hawaii was the 50th state to become apart of the United States of America.
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.
The Five-0 (5-O)had set-up a speed trap and gave me a hefty ticket!