Green’s Dictionary of Slang

certain, definite.

1849		Bell’s Life in Sydney 13 Jan. 3/2: Rejoined the Captain, ‘she must go back to her place and fulfil her engagement.’ ‘That's a moral impossibility,’ rejoined Marlha.
1889		Dead Bird (Sydney) 3 Aug. 6/3: A team of moral [i.e. certain to win] young copy improvers and machine smashers.
1889		Music Hall & Theatre Rev. 4 May 188/2: It is a moral certainty he will never get into a handicap like this.
1908		B. Fisher A. Mutt in Blackbeard Compilation (1977) 50: I got a moral cinch in the 2d race. Slip me a piece of change and I’ll take it over to the track.