Green’s Dictionary of Slang

1. to have a quick wash, to splash.

1912		H. Lawson ‘Grandfather’s Courtship’ in Roderick (1972) 856: Then he’d sloosh out and pile more native forest on the inferno in the fireplace.
1919		W.H. Downing Digger Dialects 46: sluish (vb. or n.) — Wash.

2. in fig. use, to ‘splash’ an idea around.

2001

K. Sampson Outlaws (ms.) 134: I’m going round and round with it, slooshing it round my head.