From Discworld & Terry Pratchett Wiki
Mentioned in The Truth, this street is home to Sidney Clancy (39) and Son, Purveyors of Finest Cat and Dog Meats To The Gentry. As the name suggests, it's in the slaughterhouse district, curving from Morpork Street near the rimward wall to the Cattle Market.
Hmmm. As in so many world languages "long pig" or "long hog" is a synonym for "human meat", one wonders what historical horror is hinted at here, preserved in the name of the district... is this another of those historical little byways of good ol' Ankh-Morpork that might not survive exhumation into the light?