Green’s Dictionary of Slang

grouter n.

[ety. unknown; ? Yorks. dial. grout, to rummage or root about]
(Aus.)

1. a piece of good luck, an unfair advantage; esp. as come in on the grouter

[Aus]Bulletin (Sydney) 12 Oct. 36/2: ‘Come on; heads a fiver. Now, then, Bill, get it on the grouter.’ / ‘Not me,’ says Bill; ‘I couldn’t win a blessed argument.’.
[Aus]W.H. Downing Digger Dialects 27: grouter — An unfair advantage.
[Aus](con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: grouter. The acquisition of something for nothing, or on extremely favorable terms.
[Aus]S. Gore Holy Smoke 51: He decides he’ll run a grouter on the Lord, and hook his bait.
[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 29: Grouter Person Takes advantage by coming into a venture at the end of a game.
[Aus]Macquarie Dict. 🌐 grouternoun 1. an unfair advantage.

2. as excl.

[Aus]Aussie (France) IX Dec. 4/1: gibbo: Vous tray pretty cherrie. / [Kisses Mlle.] / diggers: Grouter!

In phrases

come a grouter (v.)

(Aus.) to have an instance of bad luck.

[Aus]Sydney Sportsman (Surry Hills, NSW) 6 July 4/4: The Snail was the pea for the next, and, for fear of a grouter, I came one. and home it rolled, leaving me 15s crook.
come in on the grouter (v.)

(Aus.) to get hold of something through luck rather than judgement, to take unfair advantage of a situation.

[Aus]W.H. Downing Digger Dialects 27: ‘Come on the grouter’ — gain an unfair advantage.
[Aus]Western Argus (Kalgoorlie, WA) 15 June 1/4: ‘You come in on the grouter.’ I heard the phrase at a game of two-up.
[Aus]L. Glassop We Were the Rats 6: Ya never know. I might come in on the grouter.
[Aus](con. 1941) R. Beilby Gunner 296: By coming in on the grouter he had augmented the remaining pound of the two Whiteside had given him.
[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Grouter. 2. As in ‘come in on the grouter’ – join a venture when it is near a successful conclusion.
[Aus]Macquarie Dict. 🌐 come in on the grouter, a. to take an unfair advantage of a situation.