Green’s Dictionary of Slang

completely, utterly, totally, absolutely.

1919		Amer. Legion Weekly 12 Sept. 29: Come clean! [...] We nave got you dead bang right! [HDAS].
1933		‘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.
1953		T. Runyon In For Life 183: I caught two of them dead-bang in the act.
1979		(con. 1960) Pepper & Pepper Straight Life 261: I got busted for possession, dead bang.
1983		S. King Christine 490: They caught me dead-bang.
1997		E. Little Another Day in Paradise 257: ‘Do they got a case?’ ‘Dead fuckin’ bang, pal. Dead bang.’.
2012		T. Robinson ‘Saint of Gunners’ in Dirty Words [ebook] If whoever it was headed up the corridor, I had him dead-bang.