As drunk as David’s sow; a common saying, which took its rise from the following circumstance: One David Lloyd, a Welchman, who kept an alehouse at Hereford, had a living sow with six legs, which was greatly resorted to by the curious; he had also a wife much addicted to drunkenness, for which he used sometimes to give her due correction. One day David’s wife having taken a cup too much, and being fearful of the consequences, turned out the sow, and lay down to sleep herself sober in the stye. A company coming in to see the sow, David ushered them into the stye, exclaiming, there is a sow for you! did any of you ever see such another? all the while supposing the sow had really been there; to which some of the company, seeing the state the woman was in, replied, it was the drunkenest sow they had ever beheld; whence the woman was ever after called David’s sow.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
David Jones’s Locker * DavyNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes