1. [n]  the act of hitting vigorously; "he gave the table a whack"
2. [n]  negative criticism
3. [n]  a bad experience; "the school of hard knocks"
4. [n]  the sound of knocking (as on a door or in an engine or bearing); "the knocking grew louder"
5. [n]  a vigorous blow; "the sudden knock floored him"; "he took a bash right in his face"; "he got a bang on the head"
6. [v]  deliver a sharp blow or push :"He knocked the glass clear across the room."
7. [v]  knock against with force or violence; "My car bumped into the tree"
8. [v]  rap with the knuckles; "knock on the door"
9. [v]  of car engines, when firing too early
10. [v]  make light, repeated taps on a surface; "he was tapping his fingers on the table impatiently"

1. \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.
   {To knock off}, to cease, as from work; to desist.
   {To knock under}, to yield; to submit; to acknowledge one's
      self conquered; -- an expression probably borrowed from
      the practice of knocking under the table with the
      knuckles, when conquered. ``Colonel Esmond knocked under
      to his fate.'' --Thackeray.
   

2. \Knock\ (n[o^]k), v. t.
   1. To strike with something hard or heavy; to move by
      striking; to drive (a thing) against something; as, to
      knock a ball with a bat; to knock the head against a post;
      to knock a lamp off the table.
            When heroes knock their knotty heads together.
                                                  --Rowe.
   2. To strike for admittance; to rap upon, as a door.
            Master, knock the door hard.          --Shak.
   {To knock down}.
      (a) To strike down; to fell; to prostrate by a blow or by
          blows; as, to knock down an assailant.
      (b) To assign to a bidder at an auction, by a blow or
          knock; to knock off.
   {To knock in the head}, or {on the head}, to stun or kill by
      a blow upon the head; hence, to put am end to; to defeat,
      as a scheme or project; to frustrate; to quash. [Colloq.]
      -- {To knock off}.
      (a) To force off by a blow or by beating.
      (b) To assign to a bidder at an auction, by a blow on the
          counter.
      (c) To leave off (work, etc.). [Colloq.] -- {To knock
   out}, to force out by a blow or by blows; as, to knock out
      the brains.
   {To knock up}.
      (a) To arouse by knocking.
      (b) To beat or tire out; to fatigue till unable to do
          more; as, the men were entirely knocked up. [Colloq.]
          ``The day being exceedingly hot, the want of food had
          knocked up my followers.'' --Petherick.
      (c) (Bookbinding) To make even at the edges, or to shape
          into book form, as printed sheets.
   

3. \Knock\, n.
   1. A blow; a stroke with something hard or heavy; a jar.
   2. A stroke, as on a door for admittance; a rap. `` A knock
      at the door.'' --Longfellow.
            A loud cry or some great knock.       --Holland.
   {Knock off}, a device in a knitting machine to remove loops
      from the needles.
   

4. \Knock\, v. i.
   To practice evil speaking or fault-finding; to criticize
   habitually or captiously. [Vulgar Slang, U. S.]
   

5. \Knock\, v. t.
   To impress strongly or forcibly; to astonish; to move to
   admiration or applause. [Slang, Eng.]
   

"Though Orientals are very jealous of their privacy, they never knock when about to enter your room, but walk in without warning or ceremony. It is nearly impossible to teach an Arab servant to knock at your door. They give warning at the outer gate either by calling or knocking. To stand and call is a very common and respectful mode. Thus Moses commanded the holder of a pledge to stand without and call to the owner to come forth (Deut. 24:10). This was to avoid the violent intrusion of cruel creditors. Peter stood knocking at the outer door (Acts 12:13, 16), and the three men sent to Joppa by Cornelius made inquiry and 'stood before the gate' (10:17, 18). The idea is that the guard over your privacy is to be placed at the entrance."

Knocking is used as a sign of importunity (Matt. 7:7, 8; Luke 13:25), and of the coming of Christ (Luke 12:36; Rev. 3:20).