Green’s Dictionary of Slang

(Aus. Und.) one who backs an illegal gambling game.

1932

New Call (Perth, WA) 7 Apr. 3/4: In every ‘well-conducted’ hazard school is to be found what is known as the ‘Out-backer,’ [...] He [...] is a man of substance, and is not a gambler, in the strict sense of the term. It is not unusual for him to ‘sit behind’ a thousand or two, and he has figured out the chances long before being installed.