Three Eighths 3/8" Screw On Barbed Plastic Fitting
3/8" Screw On Barbed Plastic Fitting. It's probably for a Gerni water pressure washer, or ALTO floor scrubbing machines or sweeper.
At the moment, standard postage within Australia is capped at a maximum of $20! Maximise the saving on postage and buy some of the related parts below, such as a hose, filters for your Nilfisk or ALTO vacuum cleaner, or a new floor tool and hose.