Green’s Dictionary of Slang

1. a professional perjurer.

1725		J. Wild ‘Advice to his Successor’ in Fielding Hist. of Life of J. Wild (1840) lxii: The rapper, I think (as the cant phrase has it), is the most necessary man for your purpose.
a.1790		H.T. Potter New Dict. Cant (1795) n.p.: rapper a perjurer.
1809		G. Andrewes Dict. Sl. and Cant.
1821		Flash Dict.
c.1850		Duncombe New and Improved Flash Dict.
1886		Lantern (N.O.) 13 Oct. 3: Lottie seems to be solid with a certain gang of bums who will be always ready to act as rappers, in case of trouble, for which they receive — well, hard to say what.
1903		H. Hapgood Autobiog. of a Thief 296: The rapper seemed to weaken, the copper was less trustful and read the riot act to him.

2. (US) a plaintiff, a prosecutor.

1904		Number 1500 Life In Sing Sing 252: Rapper. Prosecutor; complainant.
1910		J. Sullivan ‘Criminal Sl.’ in Amer. Law Rev. LII (1918) 889: A complaint or charge is a ‘rap’ and the complainant is the ‘rapper.’.
1928		J. O’Connor Broadway Racketeers 254: Rapper—A complainant.
1939		Howsley Argot: Dict. of Und. Sl.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 174/2: Rapper. [...] 2. A complainant; the chief witness for the prosecution in a criminal proceeding.
1962		Ragen & Finston World’s Toughest Prison 814: rapper – The principal witness or complainant in a criminal case.

3. (US Und.) an informer.

1928		M.C. Sharpe Chicago May (1929) 61: He told Finn to bring in the rapper (squealer).
1931		(con. 1900s) C.W. Willemse Behind The Green Lights 72: If the victim proved to be a ‘rapper’ – a man who squealed to the police – the trailer gave warning in time for the girl and her confederates to disappear.

4. (US Und.) a judge.

1929		Hostetter & Beesley It’s a Racket! 227: hard rapper—Judge who imposes severe fines or sentences.
1939		Howsley Argot: Dict. of Und. Sl.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 174/2: Rapper. 1. A judge.

5. (US) one who takes or is given the blame for a crime, even if they are not actually guilty.

1940

R. Chandler Farewell, My Lovely (1949) 169: There were a couple solved for the record, but they were just rappers.

6. (US) a chatterer, a talker.

1971		Time 22 Feb. 38: Boulez clearly hopes there will be as many rappers as listeners.
1977		J. Allen Assault with a Deadly Weapon 23: Now my grandfather wasn’t no rapper. With him it was just koshk! Just grabbed you right off.

7. (US) the voice.

1969

‘Iceberg Slim’ Pimp 47: Guess how the bastard lost his ‘rapper’?

8. (US black) one who talks articulately and persuasively.

1972

Milner & Milner Black Players 236: Being a gifted talker or rapper — as we say in our world ‘he can really rap’ — he raps to those chicks and paints a kaleidoscope of color [...] a beautiful picture.