Green’s Dictionary of Slang

1. a bluebottle fly [play on bluebottle n. (2)].

1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1864, 1867, 1870		Hotten Sl. Dict.

2. (UK Und.) an informer, an untrustworthy criminal who seems over-inquisitive.

1873		Sl. Dict. 257: Policeman [...] among the dangerous classes, a man who is unworthy of confidence, a sneak or mean fellow.
1924		S. Scott Human Side of Crook and Convict Life 23: Men will listen to the vilest epithets, but call them ‘bogey’ ‘brassey,’ ‘copper’ or ‘policeman,’ and they will be at your throat.
1982		G. Newbold Big Huey 73: Anybody who went to the screws [...] was labelled a nark, a grasshopper, or a policeman.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 143/1: policeman n. 1 an inmate informer 2 a nosey person who asks many questions.

3. (US prison) a term of abuse.

1929

A.J. Barr Let Tomorrow Come 213: ‘That guy’s screwy, policeman,’ the man calls who jeered at the guard earlier in the affair.