A prison. Dr. Grey, in his notes on Hudibras, explains it to allude to one Doctor Lob, a dissenting preacher, who used to hold forth when conventicles were prohibited, and had made himself a retreat by means of a trap door at the bottom of his pulpit. Once being pursued by the officers of justice, they followed him through divers subterraneous passages, till they got into a dark cell, from whence they could not find their way out, but calling to some of their companions, swore they had got into Lob’s Pound.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Lob-1 * LobcockNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes