Source : Webster's Revised Unabridged Dictionary (1913)

Knock \Knock\, v. i. [imp. & p. p. {Knocked}; p. pr. & vb. n.
   {Knocking}.] [OE. knoken, AS. cnocian, cnucian; prob. of
   imitative origin; cf. Sw. knacka.Cf. {Knack}.]
   1. To drive or be driven against something; to strike against
      something; to clash; as, one heavy body knocks against
      another. --Bacon.

   2. To strike or beat with something hard or heavy; to rap;
      as, to knock with a club; to knock on the door.

            For harbor at a thousand doors they knocked.
                                                  --Dryden.

            Seek, and ye shall find; knock, and it shall be
            opened unto you.                      --Matt. vii.
                                                  7.

   {To knock about}, to go about, taking knocks or rough usage;
      to wander about; to saunter. [Colloq.] ``Knocking about
      town.'' --W. Irving.

   {To knock up}, to fail of strength; to become wearied or worn
      out, as with labor; to give out. ``The horses were
      beginning to knock up under the fatigue of such severe
      service.'' --De Quincey.

Knocking \Knock"ing\, n.
   A beating; a rap; a series of raps.

         The . . . repeated knockings of the head upon the
         ground by the Chinese worshiper.         --H. Spencer.

Source : WordNet®

knocking
     n : the sound of knocking (as on a door or in an engine or
         bearing); "the knocking grew louder" [syn: {knock}]