The first Monday after the school-boys holidays, or breaking up, when they are to go to school, and produce or repeat the tasks set them.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Black Legs * Black PsalmNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes