Green’s Dictionary of Slang

1. certain.

1933

J. Spenser Limey 90: That’d be a dead bang give-away.

2. (US) of a criminal prosecution, lacking any possibility of defence or argument because the case is watertight.

1935		A.J. Pollock Und. Speaks 29/2: Dead bang rap, caught red handed; a slight chance to escape conviction.
1950		Goldin et al. DAUL 56/2: Dead-bang. Red-handed; in the act. Dead-banger. An arrest in the act, or with ample evidence to convict. ‘Rubberhead Gordon got a dead-banger on a heist (holdup) down in Philly.’.
1966		R. Oliver ‘More Carnie Talk’ in AS XLI:4 279: He was caught dead bang stealing plush.
1975		J. Wambaugh Choirboys (1976) 110: They said it was a ‘dead bang’ case. A cinch.