Source : Webster's Revised Unabridged Dictionary (1913)

Pin \Pin\, n. [OE. pinne, AS. pinn a pin, peg; cf. D. pin, G.
   pinne, Icel. pinni, W. pin, Gael. & Ir. pinne; all fr. L.
   pinna a pinnacle, pin, feather, perhaps orig. a different
   word from pinna feather. Cf. {Fin} of a fish, {Pen} a
   feather.]
   1. A piece of wood, metal, etc., generally cylindrical, used
      for fastening separate articles together, or as a support
      by which one article may be suspended from another; a peg;
      a bolt.

            With pins of adamant And chains they made all fast.
                                                  --Milton.

   2. Especially, a small, pointed and headed piece of brass or
      other wire (commonly tinned), largely used for fastening
      clothes, attaching papers, etc.

   3. Hence, a thing of small value; a trifle.

            He . . . did not care a pin for her.  --Spectator.

   4. That which resembles a pin in its form or use; as:
      (a) A peg in musical instruments, for increasing or
          relaxing the tension of the strings.
      (b) A linchpin.
      (c) A rolling-pin.
      (d) A clothespin.
      (e) (Mach.) A short shaft, sometimes forming a bolt, a
          part of which serves as a journal. See Illust. of
          {Knuckle joint}, under {Knuckle}.
      (f) (Joinery) The tenon of a dovetail joint.

   5. One of a row of pegs in the side of an ancient drinking
      cup to mark how much each man should drink.

   6. The bull's eye, or center, of a target; hence, the center.
      [Obs.] ``The very pin of his heart cleft.'' --Shak.

   7. Mood; humor. [Obs.] ``In merry pin.'' --Cowper.

   8. (Med.) Caligo. See {Caligo}. --Shak.

   9. An ornament, as a brooch or badge, fastened to the
      clothing by a pin; as, a Masonic pin.

   10. The leg; as, to knock one off his pins. [Slang]

   {Banking pin} (Horol.), a pin against which a lever strikes,
      to limit its motion.

   {Pin drill} (Mech.), a drill with a central pin or projection
      to enter a hole, for enlarging the hole, or for sinking a
      recess for the head of a bolt, etc.; a counterbore.

   {Pin grass}. (Bot.) See {Alfilaria}.

   {Pin hole}, a small hole made by a pin; hence, any very small
      aperture or perforation.

   {Pin lock}, a lock having a cylindrical bolt; a lock in which
      pins, arranged by the key, are used instead of tumblers.
      

   {Pin money}, an allowance of money, as that made by a husband
      to his wife, for private and personal expenditure.

   {Pin rail} (Naut.), a rail, usually within the bulwarks, to
      hold belaying pins. Sometimes applied to the {fife rail}.
      Called also {pin rack}.

   {Pin wheel}.
       (a) A contrate wheel in which the cogs are cylindrical
           pins.
       (b) (Fireworks) A small coil which revolves on a common
           pin and makes a wheel of yellow or colored fire.

Source : WordNet®

pin grass
     n : European weed naturalized in southwestern United States and
         Mexico having reddish decumbent stems with small fernlike
         leaves and small deep reddish-lavender flowers followed
         by slender fruits that stick straight up; often grown for
         forage [syn: {redstem storksbill}, {alfilaria}, {alfileria},
          {filaree}, {filaria}, {clocks}, {pin clover}, {Erodium
         cicutarium}]