nail / nāl/ • n. 1. a small metal spike with a broadened flat head, driven typically into wood with a hammer to join things together or to serve as a peg or hook.2. a horny covering on the upper surface of the tip of the finger and toe in humans and other primates. ∎  an animal's claw. ∎  a hard growth on the upper mandible of some soft-billed birds.3. hist. a medieval unit of measurement: ∎  a measure of length for cloth, equal to 2¹/₄ inches. ∎  a measure of wool, beef, or other commodity, roughly equal to 7 or 8 pounds.• v. [tr.] 1. [tr.] fasten to a surface or to something else with a nail or nails: nail the edge framing to the wall | the teacher was nailing up the lists.2. inf. expose (someone) as deceitful or criminal; catch or arrest: have you nailed the killer? ∎  expose (a lie or other instance of deception).3. inf. Football tackle the quarterback or ballcarrier, esp. at or behind the line of scrimmage. ∎  Baseball (of a fielder) put (a runner) out by throwing to a base: he dropped a perfect throw home that should have nailed Joe by yards. ∎  (of a player) defeat or outwit (an opponent): Navratilova tried to nail her on the backhand side. ∎  (of a player) secure (esp. a victory) conclusively: there's no doubt I had chances to nail it in the last set.4. vulgar slang (of a man) have sexual intercourse with (someone).PHRASES: fight tooth and nailsee tooth.hard as nails (of a person) very tough; completely callous or unfeeling.a nail in the coffin an action or event regarded as likely to have a detrimental or destructive effect on a situation, enterprise, or person: this was going to put the final nail in the coffin of his career.PHRASAL VERBS: nail someone down elicit a firm promise or commitment from someone: I can't nail her down to a specific date.nail something down1. fasten something securely with nails.2. identify something precisely: something seems unexpected—I can't nail it down, but it makes me uneasy.3. secure something, esp. an agreement: the company has finally nailed down the agreement with its distributors.DERIVATIVES: nailed adj. [in comb.] dirty-nailed fingers. nail·less adj.