Why do people use "bone" in the phrase "bone stock" to emphasize that a car is unmodified?

It means that the car is entirely stock (no aftermarket parts at all) down to its very core. Note this definition from Merriam-Webster:

bone, noun :

a : essence, core {cut costs to the bone} {a liberal to the bone}

b : the most deeply ingrained part : heart —usually used in plural {knew in his bones that it was wrong}

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.

Never heard the phrase but my guess would be that it's a combination of 'stock' as in original and 'bare bones' as in the simplest version.

Not being a petrolhead, I wasn't familiar with this one either, but Google finds over 600K instances, so it's definitely "out there".

My guess is that for an expression like this to gain traction, it's probably picking up on multiple antecedents. Any given person may have their own idea as to why it seems reasonable, causing them to remember and pass it on.

• Bone stock is a simple base ingredient in the culinary context.

• A bare-bones computer has only basic components, well-suited to customisation.

• Bone can mean "very" (bone-tired, bone-idle), and stock can mean "standard".

Those are just the ones I can think of. There may well be others.

EDIT: I just did a bit more googling. It seems bone-stock often refers to a formal "sub-class" in organised stock car racing (like F1, F2, etc., at the higher end of car racing). The key requirement is that it should be standard, as supplied by the manufacturer (thus having "street tyres" is often the limiting factor), but they're often quite powerful vehicles. It's similar with bare-bone computers, which usually have a much more gutsy processor and motherboard than other offerings by the same supplier, so in light of that I think this origin may be more significant than the others (though they of course probably contributed to the computer usage, which I think came first).

I was hoping a search for "first use in print" on this one might offer some leads. I found the phrase used twice in the July 1973 issue of Hot Rod magazine:

The engine is made up of a bone-stock 302-cubic-inch Z-28 short block coupled. . .

And that was a comparison between bone-stock production-line versions!

However, neither offers any clear indication why bone was suddenly used to modify stock.