Eliminator
From Memory Alpha, the free Star Trek reference
An eliminator was a professional assassin in the Ferengi Alliance.
During the 2370s, Leck was an eliminator. Many (or most) Ferengi view eliminators as dangerous psychopaths, who appear to enjoy killing (thus eliminating potential customers, highly discouraged in Ferengi society) as much as or more than the profits derived thereby. (DS9: "The Magnificent Ferengi")
Considering the security requirements that Leck required Quark to use when speaking to him, it can be inferred that this is an illegal occupation amongst the Ferengi.
