A police officer or detective; (usually with the) the police.
Origin
1980s; earliest use found in The New York Times. From the name of Hawaii Five-O, a U.S. television series about a police investigative unit of the same name.
Definition of Five-O in US English:
Five-O
(also five-oh)
nounˈfʌɪvˌəʊ
US slang
A police officer or detective; (usually with the) the police.
Origin
1980s; earliest use found in The New York Times. From the name of Hawaii Five-O, a U.S. television series about a police investigative unit of the same name.