E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
a practical joke played on greenhorns aboard ship. A tub full of water is placed between two stools, and the whole being covered with a green cloth, a sailor sits on each stool, to keep the cloth tight. The two sailors represent Neptune and Amphitrit, and the greenhorn, as ambassador, is introduced to their majesties. He is given the seat of honour between them; but no sooner does he take his seat than the two sailors rise, and the greenhorn falls into the tub, amidst the laughter of the whole crew.