Green’s Dictionary of Slang

(US) a certainty.

1896		H. Blossom Checkers 235: There’s some friends of mine got a bottled-up ‘good thing’ [...] that’s a ‘mortal.’.
1945		W.R. Burnett Tomorrow’s Another Day 20: ‘Well, he won’t do it; that’s a mortal,’ said Sam, laughing again.