In British slang, a nail is an inexpensive, slow, undesirable zero status old high mileage
car.
This cheap form of disposable motoring might also be called a beater,
banger, shed, noddy
car, tub, clunker, jalopy, and, depending on origin, yank tank, eurotrash, japcrap.
Not to be confused with a lemon, which is an unroadworthy money pit you just unwisely bought.
Friend: "Mate, where did you get this old nail?"
Me: "Well, I saw it in the post office window for £
300"
Friend: "Oh. Does the
radio work?"
Me: "No. But I can fit a mattress in the back"