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

Question

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 1

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.

Answer 2

Dirt (or grit) under the fingernails or calluses on the hands are indicators of someone who actually does physical work. I think that's all that's meant. Contrast with the soft hands of what used to be called "coupon clippers"- people who collect income from passive investments.

Answer 3

Spehro identified that grit is dirt. But what's missing is to understand that your quote refers to

a little bit of grit

A little bit or a bit is a small amount or portion. It can also just mean "some"

Take a bit of bread with your soup. Take some bread with your soup.

So the writer is referring to people with (some) grit under their fingernails. Their hands are dirty. They do "real work" or work hard.