Source : Webster's Revised Unabridged Dictionary (1913)

Knock \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.

Source : WordNet®

knock off
     v 1: get rid of (someone who may be a threat) by killing; "The
          mafia liquidated the informer"; "the double agent was
          neutralized" [syn: {neutralize}, {neutralise}, {liquidate},
           {waste}, {do in}]
     2: cut the price of [syn: {shave}]
     3: take by theft; "Someone snitched my wallet!" [syn: {hook}, {snitch},
         {thieve}, {cop}, {glom}]
     4: write quickly; "She dashed off a note to her husdband saying
        she would not be home for supper" [syn: {dash off}, {toss
        off}, {fling off}]
     5: stop pursuing or acting; "drop a lawsuit"; "knock it off!"
        [syn: {drop}]