To crook one’s elbow, and wish it may never come straight, if the fact then affirmed is not true—according to the casuists of Bow-street and St. Giles’s, adds great weight and efficacy to an oath.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Crook Back * Crook ShanksNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes