Green’s Dictionary of Slang

1. (US) to talk; to complain.

1904		Number 1500 Life In Sing Sing 246: Belched, spoke.
1912		D. Lowrie My Life in Prison 216: When the commitments were sprung he belched everything he knew.
1936		Mencken Amer. Lang. (4th edn) 577: Finally, he devises many new verbs and verb-phrases or provides old ones with new meanings, e.g., to belch (to talk).
1943		R.L. Bellem ‘Feature Snatch!’ Dan Turner - Hollywood Detective Feb. 🌐 Okay, Sherlock. Start belching before I kick the truth out of you.
1949		Monteleone Criminal Sl. (rev. edn).

2. (US) to inform on.

1904		Number 1500 Life In Sing Sing 246: Belched. [...] turned informer.
1914		Jackson & Hellyer Vocab. Criminal Sl. 16: belch [...] ‘He cannot stand the gaff without belching.’.
1924		G. Henderson Keys to Crookdom 397: Belch. To talk, to turn state’s evidence, to give testimony.
1933		Ersine Und. and Prison Sl.
1942		R.L. Bellem ‘Malibu Mess’ Dan Turner – Hollywood Detective Dec. 🌐 The spy had overheard Bat Romani about to belch and had browned the chunky hood with my gat.
1950		Goldin et al. DAUL 25/2: Belch. To inform the police; to turn State’s evidence. ‘Chill (ignore) that flea (informer). He belched on his partners.’.
1951		Lipsius New Yorker 1 Dec. 106: I feel good that I didn’t belch on a pal, because that’s the code I was raised on [R].
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 8: Belch – to inform; stool; cop a plea, etc.
1992		Bentley & Corbett Prison Sl. 41: Roll Over To turn someone in to prison authorities for an illegal act he has committed […] (Archaic: belch).