Green’s Dictionary of Slang

1. a police officer.

1924

S. Scott Human Side of Crook and Convict Life 103: Some ‘splits’ (detectives) and ‘brassies’ (policemen) are exceedingly popular with crooks.

2. thus an informer.

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.

3. see brass n.¹ (1a)