Green’s Dictionary of Slang

(Aus./N.Z.) large, important, of lengthy duration etc, e.g. an old man kangaroo, an old man sand storm.

1847		[A. Harris] (con. 1820s) Settlers & Convicts 35: In one place we saw a very large opossum (in the language of the country an old man ’possum).
1861		Bell’s Life in Sydney 19 Oct. 3/1: There was a regular ‘old warrior’ of a kangaroo dog on jimbour station, that had been in at the death of many a ‘fine’ and ‘old man kangaroo’.
1890		Dead Bird (Sydney) 1 Mar. 2/4: They suddenly came upon an old-man kangaroo.
1896		E. Glanville Kloof Yarns 21: The old man baboon! He pulled the string, the door swung open, an’ he hopped in.
1901		‘Rolf Boldrewood’ In Bad Company 187: Then there’d been a fire over by the big pine-stump – an ‘old man’ fire too.
1906		E. Dyson Fact’ry ’Ands 199: Two ’underd ole-man rats that ’ad bin glued on t’Bunyip in mortil combat.
1907		Brisbane Courier 29 May 6/3: Billy was giving him an old man quilting.
1933		(ref. to 1890–1910) L.G.D. Acland Early Canterbury Runs (1951) 389: Old man – Slang. Striking, big, remarkable; e.g., an o.m. rock, flood nor’-wester, cabbage-tree, etc.
1941		C.R. Prance Tante Rebella and her Friends (1951) 95: An old-man baboon up on the koppie.
1988		McGill Dict. of Kiwi Sl. 79/1: old man a rural object of striking or large appearance; eg old man paddock or old man rock.
2003		McGill Reed Dict. of N.Z. Sl. 147: old man flood The highest flood. Late C19 old man manuka The toughest manuka log a sawyer has encountered.