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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
Buy a modern reprint of this book from Amazon CA; US;
Francis Grose was independently wealthy, having inherited money from his father, a jeweller. Finding himself overspending, he published a number of books; his Provincial Glossary seems to have been the starting-point for the Vulgar Tongue reproduced here.