A very rought type of, usually, male individual.
Nothing in particular, except the person is usually known to cause pain or to be able to handle what others would call intense amounts of it.
They often drink beer, smoke cigarrettes, and go out with sexy girls.
My neighbor was nails, he was in a car accident where his truck rolled many times without a seatbelt. He got out to smoke a cigarrette, and went and stood by the drunk man that hit him, communicating, with the man missing bodily parts and expelling massive quantities of fluid.
Only someone nails would endure such an occurance unscathed.