Green’s Dictionary of Slang

1. the badge as a symbol of being a police officer, thus gold shield, a detective.

1915		G. Bronson-Howard God’s Man 378: When you flash your shields and arrest ’em, they’ll be only too glad to make their getaways.
1935		S. Kingsley Dead End Act II: Rises, takes his pad and pencil from his pocket, glances at mulligan’s shield, writes: Mulligan...10417...19th Precinct.
1955		Q. Reynolds Police Headquarters (1956) 73: Put a shield on a man—that don’t make him honest.
1966		J. Mills Panic in Needle Park (1971) 164: The shield pays for everything in here. It may look swish, but the guys who own it are no great bargains, so they figure it’s good business to let the fuzz ride for free.
1973		P. Maas Serpico 104: Cooper [...] had come with the news Serpico had always waited for, the news that he would be promoted to detective. ‘Frank,’ he said, ‘you’re going to get the gold shield’.
1977		E. Torres Q&A 92: How come I made the gold shield in eight years and those other jerks with me in the academy are still handin’ out parkin’ tickets?
1981		H. Gould Fort Apache, The Bronx 30: That collar is makin’ him more money than your shield ever will.
1986		T. Philbin Under Cover 278: Lawless showed his shield.
1986		C. Stroud Close Pursuit (1988) 160: A gold shield [...] can be broken back to uniform with terrible speed. It’s called ‘flopped back to the bag.’.
1988		H. Gould Double Bang 237: He’ll just walk right up to the bitch, flip her the shield.
1990		D. Heilbroner Rough Justice 203: [A] fellow officer who won a gold shield because he had a ‘hook’ in the department.
2006		P. Blauner Slipping into Darkness 140: ‘Detective Ali just got his gold shield in January,’ Jimmy said meaningfully.
2012		C. Stella Rough Riders 35: No, you lummox, like a former gold shield detective knows how to catch the bad guys.
2019		D. Winslow Border [ebook] ‘[N]o drug-slinging smoke is going to murder a white New York City gold shield’.

2. by metonymy, a policeman or prison guard.

1965		B. Jackson Get Your Ass in the Water (1974) 85: He was a true disciple of the field, he never used a hammer, / or cracked it by the shield while stickin’ in the slammer.
1992		Bentley & Corbett Prison Sl. 95: Shield A policeman, prison guard or any person in the field of law enforcement who wears a badge or shield.
2006		(con. 1972) Jurgenson & Cea Circle of Six 53: [A] bunch of shields from central Harlem, all solid cops.