usage - Is “people with a bit of grit under their fingernail” an idiom, or just one-off phrase?

I was interested in the phrase, “people with a “little bit of grit under their fingernails” appearing in the New Yorker magazine’s (March 14) article titled, “American Ads, American Values.” It reads;

“In an interview with Michael McCarthy, of Ad Age, Cadillac’s
advertising director, Craig Bierley, said that “Poolside” was pitched
at consumers who earn around two hundred thousand dollars a year,
people with a “little bit of grit under their fingernails” who “pop in and out of luxury.” He also said, “These are people who
haven’t been given anything. Every part of success they’ve achieved
has been earned through hard work and hustle.”
http://www.newyorker.com/online/blogs/currency/2014/03/american-ads-american-values.html

I surmise ‘have grit” means “have courage / guts.” Though it sounds “catchy “as an ad director’s parlance, I wonder if “have a little bit of grit under one’s fingernails” is common phrase or not.

Is “under one’s fingernails” acknowledged suffix to “a bit of grit,” or just an optional use?

Answer

Grit under the fingernails is a fairly standard American expression. It means that the hands have gotten dirty by performing manual labor.

The notion of the $200K salary mark equating with "a little bit of grit under the fingernails", is a cute one. What he's really implying is that these are upwardly mobile people who have worked their way up from manual laborers to earners. And, that they haven't quite cleaned the grit out of their fingernails, yet. (And, might even still do some manual labor when necessary.) These would be people who are now managers of people who do the type of work they used to do. Say, mid-level managers in an auto-factory.

This nuanced version of the original expression is not a standard expression. It requires the context and exposition around it to make people understand what it means.