Green’s Dictionary of Slang

dead-bang adv.

[dead adv. + bang adv. (1)]

completely, utterly, totally, absolutely.

[US]Amer. Legion Weekly 12 Sept. 29: Come clean! [...] We nave got you dead bang right! [HDAS].
[US]‘Goat’ Laven Rough Stuff 129: They knew we were working and wanted to break up the ring, but they couldn’t just get us dead bang.
[US]T. Runyon In For Life 183: I caught two of them dead-bang in the act.
[US](con. 1960) Pepper & Pepper Straight Life 261: I got busted for possession, dead bang.
[US]S. King Christine 490: They caught me dead-bang.
[US]E. Little Another Day in Paradise 257: ‘Do they got a case?’ ‘Dead fuckin’ bang, pal. Dead bang.’.