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. For example: "I thought she was as hard as nails when I met her." or "He simply looked mean and hard as nails." A nail is a thin piece of metal with one pointed end and one flat end. You hit the flat end with a hammer to push the nail into something such as a wall.

Source: Collins