Dutch, not an uncommon nickname, seems to be used for a variety of reasons. It could be because a person is connected to Holland, or because a person is of German descent. Here, "Dutch" is from "Deutsch," the German-language word for "German." Other reasons are more idiosyncratic. Here are some examples:
Ronald Reagan, 40th President of the United States
As a boy, Reagan's father nicknamed his son "Dutch", due to his "fat little Dutchman"-like appearance, and his "Dutchboy" haircut; the nickname stuck with him throughout his youth.
Dutch Schultz, notorious gangster
His old associates dubbed him "Dutch" Schultz in honor of a deceased strong-arm thug who was notorious for dirty fighting at the turn of the century.
Dutch Ruppersberger, U.S. congressman
Dutch is legally part of his name: Charles Albert Dutch Ruppersberger III, but it wasn't always. When Rep. Charles Albert Ruppersberger III (D-Md.) first ran for Congress in 2002, he decided that his 13-letter last name was too long to put on a bumper sticker.
Besides, he says, he needed something catchier, something that would stick in people’s heads. So he opted for his lifelong nickname: Dutch. But there was one hitch: It wasn’t his real name, so he couldn’t put "Dutch" on the ballot.
"When you market yourself, you have to make sure that you have the same name on the ballot, so I needed ‘Dutch’ on the ballot," he said in an interview. "So what I did is I legally -- I’m a lawyer -- I legally added Dutch to my name. So I would go by C -- period -- A -- period -- Dutch, and all of the bumper stickers would say, ‘Go Dutch.’ So legally I added Dutch to my name."
But why did he choose "Dutch"? The Maryland Democrat’s nickname has been --Dutch-- since the moment of his birth.
--When I was born ... the doctor came out and said to my father, ‘You have a big blond Dutchman,’ -- he said. --So they started calling me ‘Dutch,’ and when my mother and he would write letters, he’d ask, ‘How’s the Dutchman doing?’ I’ve been called Dutch all of my life.--
Dutch Fehring, Stanford's winningest baseball coach.
It was during his freshman year in high school that he earned the nickname --Dutch,-- because he was of German descent. After Fehring returned a kickoff 60 yards for a touchdown, a local sportswriter tabbed him --The Flying Dutchman-- and the nickname stuck.
Best Answer
It means that the car is entirely stock (no aftermarket parts at all) down to its very core. Note this definition from Merriam-Webster:
It's tied in with other bone-related idioms like "close to the bone," "feel something in one's bones," "chilled to the bone," and the like.