Green’s Dictionary of Slang

(Aus./US) a general expression of agreement or approval.

1859		Matsell Vocabulum 76: rybuck All right; straight; it will do; I am satisfied.
1895		Truth (Sydney) 20 Jan. 4/7: ‘He’s Sir Roger Tichborne, his very self,’ says the old dowager. ‘Ri-buck?’ says I. ‘Fair dinkum,’ says she.
1896		Barrier Miner (Broken Hill, NSW) 31 Dec. 2/8: I made known my mission. ‘Ryebuck!’ he retorted.
1901		Bulletin (Sydney) 7 Dec. 27/2: The Bench half-heartedly admitted that it did. / ‘Ryebuck, then! Many’s the flamin’ good quid I’ve blued in your ’ouse – more’n ’e’ll ever spend. Aint I?’ / Further reluctant admission from the Bench.
1901		W.S. Walker In the Blood 115: ‘Arn’t us, Loo, my beauty?’ ‘Ri-buck, ole man, so we is,’ responded Loo.
1908		Sporting Times 27 June 1/4: All they [three Sydney larrikins] could say was ‘Ryebuck!’ (when they wor pleased), ‘Straight wire?’ (when they wor doubtful), and ‘Blimy!’ (when they wor disgusted).
1911		L. Stone Jonah 11: ‘Rye buck!’ cried Jonah.
1918		C.J. Dennis ‘Got-A-Fag’ in Backblock Ballads 31: An apoplectic colonel gasped, ‘Of all the dashed infernal’.... / As this Private Smith saluted, with ‘Ribuck, boss! Got a fag?’.
1924		G.H. Lawson Dict. of Aus. Words And Terms 🌐 RIBUCK — Expression of assent.
1933		N. Lindsay Saturdee 168: She’d say ‘Rybuck’ or somethin’.
1940		E. Curry Hysterical Hist. of Aus. 110: Lefty Buggins thanks im an orl that fer is nise invertashun an ses ribuck ole pal Ile cum wiv bels on.
1968		S. Gore Holy Smoke 79: ‘Actually she’s a half-squarie –’ ‘What they call a bag-swinger, down in the city?’ ‘Ribuck, mate!’.
1996		B. Scott Banshee and Bullocky 12: The old bloke asked Arch if he could camp there [...] Arch said ‘Ribuck’, an expression he’d picked up in France in 1917.