The Collaborative International Dictionary of English v.0.48:

Nail \Nail\ (n[=a]l), n. [AS. naegel, akin to D. nagel, OS. &
   OHG. nagal, G. nagel, Icel. nagl, nail (in sense 1), nagli
   nail (in sense 3), Sw. nagel nail (in senses 1 and 3), Dan.
   nagle, Goth. ganagljan to nail, Lith. nagas nail (in sense
   1), Russ. nogote, L. unguis, Gr. "o`nyx, Skr. nakha.
   [root]259.]
   1. (Anat.) the horny scale of plate of epidermis at the end
      of the fingers and toes of man and many apes.
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            His nayles like a briddes claws were. --Chaucer.
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   Note: The nails are strictly homologous with hoofs and claws.
         When compressed, curved, and pointed, they are called
         talons or claws, and the animal bearing them is
         said to be unguiculate; when they incase the
         extremities of the digits they are called hoofs, and
         the animal is ungulate.
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   2. (Zool.)
      (a) The basal thickened portion of the anterior wings of
          certain hemiptera.
      (b) The terminal horny plate on the beak of ducks, and
          other allied birds.
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   3. A slender, pointed piece of metal, usually with a head[2],
      used for fastening pieces of wood or other material
      together, by being driven into or through them.
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   Note: The different sorts of nails are named either from the
         use to which they are applied, from their shape, from
         their size, or from some other characteristic, as
         shingle, floor, ship-carpenters', and horseshoe nails,
         roseheads, diamonds, fourpenny, tenpenny (see Penny,
         a.), chiselpointed, cut, wrought, or wire nails, etc.
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   4. A measure of length, being two inches and a quarter, or
      the sixteenth of a yard.
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   Nail ball (Ordnance), a round projectile with an iron bolt
      protruding to prevent it from turning in the gun.

   Nail plate, iron in plates from which cut nails are made.


   On the nail, in hand; on the spot; immediately; without
      delay or time of credit; as, to pay money on the nail; to
      pay cash on the nail. "You shall have ten thousand pounds
      on the nail." --Beaconsfield.

   To hit the nail on the head,
      (a) to hit most effectively; to do or say a thing in the
          right way.
      (b) to describe the most important factor.
          [1913 Webster +PJC]