Green’s Dictionary of Slang

dead n.2

In phrases

on the dead [abbr. on the dead (level) under dead adj.]

(US) in earnest, sincerely, straightforwardly, honestly.

[US]H. Blossom Checkers 36: Say, on the dead, I’m glad to see you.
[US]A.H. Lewis ‘Politics’ in Sandburrs 96: On d’ dead! I was farmer enough to t’ink I’d t’ank him for bein’ me guide.
[US]S. Ford Shorty McCabe 198: On the dead now, I don’t care so much about loomin’ up in the picture.
[US]Wash. Post 28 June 6/6: But, say, my picture, on the dead, / Ain’t been in any pape.
[US]Wentworth & Flexner DAS.