Why is it called Stock Car?

[Mr 5 0]

Yep; way, way back in the early 1950's they (NASCAR) were running regular, production-bodied cars like Olds 88's with 303 cu. in. V-8 engines and even Hudson Hornets and V-8 Caddys, believe it or not.
That didn't last long as reliability was a problem and little by little, the cars were less and less stock and more and more race cars. By the early '60's, they were just about all race car, although they still used factory parts in many cases.
Even now, the term 'stock car' refers to a car that at least resembles a factory production vehicle, although everyone knows it's not anything remotely close to a factory car, any more than that trailered, 7-second twin turbocharged bored and stroked 351 Mustang at the drag strip resembles a 'factory' Mustang. It's just a symbol for the fans to root for, be it Chevy, Ford or whatever. Works for me.