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He has as much need of a wife as a dog of a side pocket; said of a weak old debilitated man. He wants it as much as a dog does a side pocket; a simile used for one who desires any thing by no means necessary.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Sick As a Horse * SidledywryNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes
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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.