Five-O is a slang term that means law enforcement or police. The term comes from the old late-1960's police procedural television series Hawaii Five-O starring Jack Lord. It was at one point the longest running police procedural in television history until Law and Order took the crown.
The slang tends to be used by folks who do not look at law enforcement officers as helpful or on their side. It crops up often in criminal dialog in scripts and fiction, with a slant towards black gang members who are up to no good. The trope is furthered by the jargon, and counts on racial stereotypes to solidify the character.
Iron Noder 2017