E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Drunk as Davys sow. Grose says: One David Lloyd, a Welshman, who kept an ale-house at Hereford, had a sow with six legs, which was an object of great curiosity. One day Davids wife, having indulged too freely, lay down in the sty to sleep, and a company coming to see the sow, David led them to the sty, saying, as usual, There is a sow for you! Did you ever see the like? One of the visitors replied, Well, it is the drunkenest sow I ever beheld. Whence the woman was ever after called Davys sow. (Classical Dictionary of the Vulgar Tongue.)