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