The Collaborative International Dictionary of English v.0.48:

Knock \Knock\ (n[o^]k), v. i. [imp. & p. p. Knocked (n[o^]kt);
   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.
      [1913 Webster]

   2. To strike or beat with something hard or heavy; to rap;
      as, to knock with a club; to knock on the door.
      [1913 Webster]

            For harbor at a thousand doors they knocked.
                                                  --Dryden.
      [1913 Webster]

            Seek, and ye shall find; knock, and it shall be
            opened unto you.                      --Matt. vii.
                                                  7.
      [1913 Webster]

   3. To practice evil speaking or fault-finding; to criticize
      habitually or captiously. [Slang, U. S.]
      [Webster 1913 Suppl.]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Knock \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.
      [1913 Webster]

            When heroes knock their knotty heads together.
                                                  --Rowe.
      [1913 Webster]

   2. To strike for admittance; to rap upon, as a door.
      [1913 Webster]

            Master, knock the door hard.          --Shak.
      [1913 Webster]

   3. To impress strongly or forcibly; to astonish; to move to
      admiration or applause. [Slang, Eng.]
      [Webster 1913 Suppl.]

   4. To criticise; to find fault with; to disparage. "Don't
      knock it if you haven't tried it."
      [PJC]

   To knock in the head, or To knock 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.
      (d) To make pregnant. Often used in passive, "she got
          knocked up". [vulgar]

The Collaborative International Dictionary of English v.0.48:

Knock \Knock\, n.
   1. A blow; a stroke with something hard or heavy; a jar.
      [1913 Webster]

   2. A stroke, as on a door for admittance; a rap. " A knock at
      the door." --Longfellow.
      [1913 Webster]

            A loud cry or some great knock.       --Holland.
      [1913 Webster]

   Knock off, See knock off in the vocabulary.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

backfire \backfire\, back fire \back fire\
   1. A fire started ahead of a forest or prairie fire to burn
      only against the wind, so that when the two fires meet
      both must go out for lack of fuel.
      [Webster 1913 Suppl.]

   2.
      (a) A premature explosion in the cylinder of a gas or oil
          engine during the exhaust or the compression stroke,
          tending to drive the piston in a direction reverse to
          that in which it should travel; also called a knock
          or ping.
      (b) an explosion in the exhaust passages of an internal
          combustion engine.
          [Webster 1913 Suppl. +PJC] Backfire

WordNet (r) 3.0 (2006):

knock
    n 1: the sound of knocking (as on a door or in an engine or
         bearing); "the knocking grew louder" [syn: knock,
         knocking]
    2: negative criticism [syn: knock, roast]
    3: a vigorous blow; "the sudden knock floored him"; "he took a
       bash right in his face"; "he got a bang on the head" [syn:
       knock, bash, bang, smash, belt]
    4: a bad experience; "the school of hard knocks"
    5: the act of hitting vigorously; "he gave the table a whack"
       [syn: knock, belt, rap, whack, whang]
    v 1: deliver a sharp blow or push :"He knocked the glass clear
         across the room" [syn: knock, strike hard]
    2: rap with the knuckles; "knock on the door"
    3: knock against with force or violence; "My car bumped into the
       tree" [syn: bump, knock]
    4: make light, repeated taps on a surface; "he was tapping his
       fingers on the table impatiently" [syn: tap, rap,
       knock, pink]
    5: sound like a car engine that is firing too early; "the car
       pinged when I put in low-octane gasoline"; "The car pinked
       when the ignition was too far retarded" [syn: pink, ping,
       knock]
    6: find fault with; express criticism of; point out real or
       perceived flaws; "The paper criticized the new movie"; "Don't
       knock the food--it's free" [syn: knock, criticize,
       criticise, pick apart] [ant: praise]