Green’s Dictionary of Slang

1. a pursuer, an informer.

1724		Defoe Remarkable Life of John Sheppard 25: Upon Sheppard’s seeing Langley a Turnkey at Newgate, he says to his Companion Page, I see a Stag .
1725		New Canting Dict. n.p.: stag a Term (inverting Qualities) used for an Enemy, a Pursuer; as, I spy a Stag, used by that notorious young Robber Shepherd, lately executed, when he first saw the Turnkey of Newgate, who pursu’d him and took him after his first Escape from the Condemn’d Hold.
1764		Bloody Register II 299: Upon Sheppard’s seeing Mr. Langley, one of the head turnkeys of Newgate, he says [...] I spy a stag.
a.1790		H.T. Potter New Dict. Cant (1795) n.p.: stagg an accomplice who has actually become a king’s evidence.
1809		G. Andrewes Dict. Sl. and Cant [as cit. a.1790].
1833		‘The Slap-Up Cracksman’ in Swell!!! or, Slap-Up Chaunter 42: Who would be a lob or stag? / And splitting, wag his d—d red rag.
1845		W. Carleton ‘Bob Pentland’ Tales and Stories of the Irish Peasantry (1854) 53: The two worst informers against a private [poteen] distiller, barring a stag, is a smoke by day, an’ a fire by night.
1859		Matsell Vocabulum 85: stag One who has turned State’s evidence.
1862		Western Times 26 July 7/6: The culprit, in the slang of felony, ‘died game,’ rejoicing that he had not proved himself a ‘stag’.
1894		Shields Dly Gaz. 11 Apr. 3/2: ‘There must have been a stag (informer) among us. How else could the police have known [etc.]’.
1895		Crowe Aus. Sl. Dict. 80: Stag, an informer.
1901		Argus (Melbourne) 20 Sept. 6/4: [If] he works in company he has an anxious time lest some of the stags or snitchers may chirp or cackle, squeak or whiddle, if hush stuff is not forthcoming or, to put it in plainer English, informers may speak unless they are paid for their silence.
1908		Warminster & Westbury Jrnl 18 Apr. 3/2: ‘They are “stags,” don’t you see. They inform’.
1910		P.W. Joyce Eng. As We Speak It In Ireland (1979) 334: Stag; an informer, who turns round and betrays his comrades.
1943		G.A. Little Malachi Horan Remembers 127: They say that Doyle not only did the stag on Emmet but tried to arrest him himself.
1956		B. Behan Quare Fellow (1960) Act II: And have his mate in the Bridewell, before the day was out. I know you, you bloody old stag.

2. a man who attends courts in order to hire himself out as a defence witness, usu. to provide an alibi for an otherwise guilty defendant.

1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 165: Queer bail are ‘stag:’ those men who being hired at a guinea or two per oath, to swear they are worth vast sums, stand about the judges’ chambers in term-time, giving out cards of address, which are commonly their whole stock in trade.
1848		Bartlett Dict. Americanisms 329: stag. In the New York courts, a stag is the technical name for a man who is always ready to aid in proving an alibi, of course ‘for a consideration’.

3. (US) a detective.

1932

Dothan Eagle (AL) 13 Dec. 2/4: Stag— a detective.