Potatoes. It is a common joke against the Irish vessels, to say they are loaded with fruit and timber, that is, potatoes and broomsticks. Irish assurance; a bold forward behaviour: as being dipt in the river Styx was formerly supposed to render persons invulnerable, so it is said that a dipping in the river Shannon totally annihilates bashfulness; whence arises the saying of an impudent Irishman, that he has been dipt in the Shannon.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Jowl * Irish BeautyNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes