Thick legs, jocularly styled the Irish arms. It is said of the Irish women, that they have a dispensation from the pope to wear the thick end of their legs downwards.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Irish Evidence * Irish ToylesNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes