Green’s Dictionary of Slang

a quick wash.

1893		H. Lawson ‘The Darling River’ in Roderick (1972) 87: A rouseabout [...] got into into his head that a good ‘sloosh’ would freshen him up.
1900		Bulletin (Sydney) 12 May 14/1: The ‘Combination’ Kitchen – Vide Plague Report. / Mr. Wayback [...]: ‘Would you mind showing me where the kitchen is?’ / Housemaid (a little curious): ‘What do you want the kitchen for?’ / Mr. Wayback: ‘Well, yer see, I’d like to give mesel’ a real good sloosh!’.
1912		H. Lawson ‘Grandfather’s Courtship’ in Roderick (1972) 861: I went home dog-tired, and had a slush and went in and sat down to dinner.
1919		Athenaeum 11 July 582/2: Among the brand-new slang one may discern some that had an onomatopœic or at any rate an imitative origin; for instance ‘sloosh’, a wash.
1919		W.H. Downing Digger Dialects 46: sluish (vb. or n.) — Wash.
1924		(con. WWI) A.G. Pretty Gloss. Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: sloosh. A wash. Corruption of ‘Sluice.’.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 261: Sloosh: A wash.