Nail  —n. 1 small metal spike hammered in to join things together or as a peg or decoration. 2 horny covering on the upper surface of the tip of the human finger or toe. —v. 1 fasten with a nail or nails. 2 secure or get hold of (a person or thing). 3 keep (attention etc.) Fixed. 4 expose or discover (a lie or liar).  nail down 1 bind (a person) to a promise etc. 2 define precisely. 3 fasten (a thing) with nails. Nail in a person's coffin something thought to increase the risk of death. On the nail (esp. Of payment) without delay. [old english]