According to Wikipedia an idiom is "a term or phrase whose meaning cannot be deduced from the literal definitions and the arrangement of its parts, but refers instead to a figurative meaning that is known only through common use."
An overused idiom is a cliche, and do we ever have cliches in genealogy! Here are a couple of my (un)favourites.
Search brickwall on Google and the fourth hit is an article from ProGenealogists which describes brickwalls as "those genealogical research problems that seem too hard to solve." There is a Yahoo forum where you can post your brickwalls. Family Chronicle magazine has published 500 Brickwall Solutions to Genealogy Problems and More Brickwall Solutions to Genealogy Problems.
Putting flesh on the bones means adding detail so as to make something more interesting. To means going beyond genealogy, the recitation of names, dates and relationships, to learning about the person's life as described in this article by Sher Leetooze on the Ontario Genealogical Society owned website "Generations, Memorial that live on." Incidentally, I hadn't encountered that web site previously. It has content, such as how to plan and funeral and a directory of florists, that seems a bit unusual for a genealogical society.
There must be many others. "Digging up you roots" comes to mind.