13 June 2020
Five-oh (or five-o) and twelve are two numerical, Black, slang designations for the police. Both appear, at least at first glance, to come from the titles of old U.S. television series: Hawaii Five-O (1968–80, rebooted 2010–present) and Adam-12 (1968–75).
Of the two, five-oh is older and the origin more certain. It appears in print in a 29 August 1983 New York Times article, although it is older in oral use:
At many parks and corners, a detective is called a “D.T.” On the Upper West Side, a “Five-O” refers to a uniformed police officer. The word comes from the television police series “Hawaii Five-O.”
By the time five-oh appears in print, the television series had been off the air for three years but could still be readily seen in syndication. The number in the TV series title comes from Hawaii being the fiftieth state—it had been a state for less than ten years and its place in the union still somewhat novel when the series premiered.
The connection of twelve to the Adam-12 series is more tenuous. The slang term is first documented in a 6 May 2003 submission to Urban Dictionary:
twelve
the police.
aye folk there go twelve right there.
In the case of twelve, the series had been off the air for over twenty-five years, and it wasn’t commonly in syndication, although it perhaps could be seen on retro cable-TV channels. It may have been deliberately coined on the model of five-oh, only picking a different police show. But such deliberate coinages are rarely successful, and it seems likely that the twelve is actually a reference to something else, and the TV-series is just an after-the-fact rationalization by someone familiar with the five-oh designation.
Sources:
Breslin, Rosemary. “City Teen-Agers Talking Up a ‘Say what?’ Storm.” New York Times, 29 August 1983, B2. ProQuest Historical Newspapers.
Green’s Dictionary of Slang, 2020, s.v. five-oh n., twelve n.
Oxford English Dictionary, third edition, June 2006, s.v. Five-O, n.
Urban Dictionary, 6 May 2003, s.v. twelve.