Green’s Dictionary of Slang

1. (also cooper) to arrest.

1858		A. Mayhew Paved with Gold 342: We shall be coopered if we don’t pad the hoof, and that rapid.
1872		E. Crapsey Nether Side of NY 17: Safe-breakers [...] are less successful than the other classes, and are more frequently ‘coppered’—that is, arrested.
1910		Spokane Press (WA) 13 Mar. 17/2: Policeman Brown was a foxy cop / [...] / Behind a tree he’d lie in wait / [...] / He’d copper the fast and copper the slow, / Then up before the judge he’d go.

2. (US) to discover.

1907		J. London Road 176: Our intention was to take the first train out, but the railroad officials ‘coppered’ our play — and won.
1908		‘Old Sleuth’ Dock Rats of N.Y. (2006) 99: The detective walked out. He had ‘coppered’ all he required for the time being.

3. to inform; thus coppering n.

1924		S. Scott Human Side of Crook and Convict Life 23: ‘Coppering,’ or turning informant, is the deadly sin among crooks.
1962		B. McGhee Cut and Run (1963) 173: Bandy knew, deep down, that he could never ‘copper’ Ben. He had never shopped anyone in his life.

4. to work as a police officer or detective; thus coppering n.

1923		D. Hammett ‘House Dick’ Nightmare Town (2001) 42: I had three days of hotel-coppering while a man was being found to take the job permanently.
1974		G.F. Newman You Flash Bastard 165: They were now experiencing a new form of coppering, the kind which carried with it the responsibility of higher office.