The Smiling Partner was a mythological figure in the Ferengi civilization, akin to the Bogeyman of Earth, the Nameless Traitor of Cardassia, and the Klingons' Keeper of the Dishonored Dead. The Smiling Partner was a character who behaved as a friend and offered what seemed to be a fair and profitable deal. Once contracts were signed, the Smiling Partner would bite off its victim's ears, and laugh as they died.
In one version of "The Story of Ving and Ding", the Smiling Partner is responsible for the deaths of Ving and Ding's parents.
Quark cited the Smiling Partner to illustrate the 48th Rule of Acquisition, "The bigger the smile, the sharper the knife." (DS9 novel: Legends of the Ferengi)
In the year 2374, Quark likened the emblem of the Cardassian Union to being a cross between the Smiling Partner and a screwdriver. (DS9 - Millennium novel: The Fall of Terok Nor)