A trick to duck some ignorant fellow or landsman, frequently played on board ships in the warm latitudes. It is thus managed: A large tub is filled with water, and two stools placed on each side of it. Over the whole is thrown a tarpaulin, or old sail: this is kept tight by two persons, who are to represent the king and queen of a foreign country, and are seated on the stools. The person intended to be ducked plays the Ambassador, and after repeating a ridiculous speech dictated to him, is led in great form up to the throne, and seated between the king and queen, who rising suddenly as soon as he is seated, he falls backwards into the tub of water.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Altitudes * Ambassador of MoroccoNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes