If you say that someone has hit the nail on the head, you think they are exactly right about something. `I think it would civilize people a bit more if they had decent conditions.' --- `I think you've hit the nail on the head.'
a thin pointed piece of metal that is hammered into materials as a fastener horny plate covering and protecting part of the dorsal surface of the digits a former unit of length for cloth equal to 1/16 of a yard attach something somewhere by means of nails; "nail the board onto the wall
a nail in the coffin: see coffin to fight tooth and nail: see tooth. In construction and carpentry, a slender metal shaft, pointed at one end and flattened at the other end, used as a fastener. Most often used to join pieces of wood, nails are also used with plastic, drywall, masonry, and concrete. They are usually made of steel but can also be made of stainless steel, iron, copper, aluminum, or bronze. The pointed end is called the point, the shaft is called the shank, and the flattened part is called the head. Structure made of keratin that grows on the back of the end of each finger and toe. Nails consist of a root, under the skin; a translucent plate, attached to a nail bed beneath; and a free edge. Nail plate (and probably bed) cells are produced at the root and pushed forward as new cells form behind them. They die and turn white as they reach the free edge and lose contact with the richly vascularized nail bed, which supplies them with nutrients. Fingernails grow continuously at about 0.5 mm per week; toenails grow more slowly. They protect the tips of fingers and toes and help the fingers pry, pick up small objects, and scratch
If you say that someone is as hard as nails, you mean that they are extremely tough and aggressive, either physically or in their attitude towards other people or other situations. He's a shrewd businessman and hard as nails
succeed at easily; "She sailed through her exams"; "You will pass with flying colors"; "She nailed her astrophysics course"