Newgate or any other prison. New College: the Royal Exchange. King’s College: the King’s Bench prison. He has been educated at the steel, and took his last degree at college; he has received his education at the house of correction, and was hanged at Newgate.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Collar Day * College CoveNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes