Green’s Dictionary of Slang

1. the act of informing.

1929		Hostetter & Beesley It’s a Racket! 238: […] ‘Singing’—the act of confessing.
1941		D. Burley N.Y. Amsterdam News 18 Jan. 20: Pryor, convicted killer of ‘Blue’ Renier, did some singing [...] confessing his part in the crime and naming two others as accomplices!!!
1982		G.V. Higgins Patriot Game (1985) 56: He was taking up singing, like you said, but the only guy he had a real chance of making, far as I could see, was some fence in Millis that handled mostly hot washing machines.

2. attrib. use of sense 1.

1949		Monteleone Criminal Sl. (rev. edn) 209: singing school A third degree; the rooms at police headquarters in which confessions are obtained.
1963		A.S. Fleischman Venetian Blonde (2006) 256: If the cops get hold of me, I may develop a singing voice.

3. (US gay) a sustained verbal attack.

1972

B. Rodgers Queens’ Vernacular 181: singing long verbal assault; stinging lecture.