Welcome to you, Seona; but I'm afraid I can't help you with the origin. I am looking at it in a dictionary, though, that was given me by my good friend jmh.
skoosh, scoosh verb gush in spurts or splashes, squirt; dart or move quickly with a swishing sound.
noun 1 a splash, spurt, jet (of liquid). 2 lemonade etc.
adverb with a splash or swish.
skoosher a device for sprinkling or spraying, a sprinkler.

This is from the "Pocket Scots Dictionary", put out by the Scottish National Dictionary Association, isbn # 1-902930-02-9. (Note: having triple-checked, I feel safe in saying that I have been accurate in re-creating all punctuation.)