What does "five O" mean (and why)?
Solution 1:
It comes from the American police show Hawaii Five-O; "five-o" has since become a slang term for police. In the show, "Five-O" is just a police unit, but the name itself doesn't really mean anything other than a designation (I believe the writers named it "Five-O" in homage to Hawaii, the 50th US state and the location of the show).
Solution 2:
From the old cop show Hawaii Five-O, sadly enough (which in turn got its name from the fact that Hawaii was the 50th state to join the union).
I wonder if cops in Cleveland bust down the door yelling “Ohio One-Seven”?