Two giants, whose effigies stand on each side of the clock in Guildhall, London; of whom there is a tradition, that, when they hear the clock strike one, on the first of April, they will walk down from their places.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Gog * GogglesNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes