Supposed by the vulgar to be more binding than an oath taken on the Testament only, as being the bigger book, and generally containing both the Old and New Testament.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Bible * BiddyNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes