Persons chained or handcuffed together, in order to be conveyed to gaol, or on board the lighters for transportation, are in the cant language said to be married together.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Marriage Music * MarrowbonesNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes